Let me list a few ideas:
Metal products. It is very unlikely that there would be many merfolk willing to spend prolonged periods of time outside of water just to craft metal objects. Metal products can be many, from weapons(most likely harpoons, or other weapons that are meant to stab) to tools, jewelry and ornaments. While rare, you don't need to use much ...
Since you have given us only "the mesopelagic zone" to go on as far as what your mermaid physiology, we will have to make the following assumptions:
The mermaid probably has large sensitive eyes in order to see in the twilight and bioluminescent environment.
The mermaid is not a mammal. (no seashells, sorry)
The mermaid is cold blooded.
The mermaid has ...
Use Coral for the Spires
You could have a giant or fast growing coral that you either use as building material
(coral can be as hard as concrete) or use it as the building itself, that could look cool as well.
A joint Building Project
The mer could also get help from the humans to build the top of the sea-spires with the mer building the bottom. They ...
Currently, static oil rigs built on reinforced concrete towers are built in sea depths of up to 350 metres. This isn't the limit of the structural mechanics, but more of an economic limit.
The deeper concrete structures are built using concave towers (thicker at the bottom and the top). The thicker top actually provides some buoyancy reducing the force on ...
I'm gonna go out on a limb here and say no (and get away with it because you haven't asked for hard science!)
$N_2$ is smaller and lighter than $CO_2$, and neither are polar. There's no trivial way to make a membrane that would let the larger molecule through and reject the smaller. Technology does exist to filter nitrogen (used in oxygen concentrators) and ...
Many of the ideas I had (metal goods, ceramics, etc) are well-described by other answers. So I'll focus on the ones that are not yet listed.
Even on land, secure storage of goods is an important service. Banks can secure actual goods (think safe deposit boxes) or virtual ones (a tally of your money, so you don't have to hide it under your ...
Ancient Rome's Underwater Concrete
More than 2000 years ago, the Roman Empire invented a unique marine
concrete that allowed for the construction of enormous, durable
structures – even underwater. Incredibly, the exact chemical
properties of this concrete mixture have eluded scientists to this day
– but now, researchers from the University of Utah ...
It's blue, Sir
Since most people can't barely hold a minute, and 99% of the population won't exceed a mere five minutes, they're dead. Very very dead. (if you take 10-ish minutes). The duration for reanimation after loss of consciousness and oxygen is around 3 minutes if I remember correctly. So barring the pressure problem necessiting a full analysis (that ...
What They Have...
Medicinal Underwater Plants
Psychotropic Underwater Plants
Transportation (storm-proof undersea barges)
Coral and Geodes
What They Want...
Rustproof Metal Tools and Weapons
Bags/Jugs of fresh water
Buoyant wood - it is like anti-gravity to them
Swim beneath their ships and bore holes in them long before they're anywhere near your balloons, or even do it in their ports. 20mph is plenty of speed to accomplish this and ships are not cheap, they wouldn't lose many before deciding it wasn't worth the effort unless your cities are full of gold, and even then they have no way of fighting underwater.
Assuming you're asking about human skin, not a fictional blubber.
It depends on the depth at which they live
Melanin (a brown pigment found in human skin) is a natural form of sunscreen, it is also the only colouring in human skin (pinker skin comes from oxygenated blood near the surface of the skin).
The graph below shows the absorption of different ...
It's Black, really, really black
There is rarely any significant light beyond 200 meters. NOAA
While it is possible they may be alive (not technically brain dead - yet) they, at the very least, if conscious, are going to be in great pain and probably convulsing.
What happens to the body at 70 bar?
P1V1 = P2V2
V2 = V1P1/P2
Every airspace in the body ...
Those bones are big. They can be found on the ocean floor.
Ancient Siberians made huts out of mammoth bones.
And people do make huts out of whalebone found on the beach.
Probably there would be a lot more whale bones available to your merpeople ...
Don't think too hard - just look at real life
In a way you kind of answered your own question. In real life we have lots of countries that buy/sell each other loads of things. In the past, we had barter, but many governments minted their own coin. This goes back to at least Ancient Greek times - the Chinese even had paper currency!
Alternatively, larger ...
A pair other ideas about what merfolks could desire from surface people:
air: surface people could easily fill animal bladders or goatskins
with air (they have lungs) and sell them to the merfolks: they could be used as
entertainment (think of bubble fountains for the VIP parties) or to help lifting heavy weights underwater. Of course, if merfolk have lungs ...
Giant Kelp forests around your settlements
have a massive kelp forest wall around your settlements were your Mer people hide traps that sunk or damage ships, launch surprise ambushes and conceal your balloons from the attackers. The traps could range from simple drift wood spikes to burning barrels from whale oil. Your Mer people can swim in and out of the ...
You could do this with a cross between a SCUBA system and an oxygen concentrator.
Oxygen concentrators typically use pressure swing adsorption (PSA)
technology and are used very widely for oxygen provision in healthcare
applications, especially where liquid or pressurized oxygen is too
dangerous or ...
I want to frame-challenge the notion that merfolk could not manufacture materials that require being on dry land to make.
humans run refineries that are hundreds of feet tall, yet we do not have wings
humans run fish hatcheries, yet we do not have gills
humans run steel mills, yet we are not fire elementals
Just like all those things have catwalks or ...
I like the Coral husbandry of JBH and carving out of cave/cliff walls. But I can add one more option.
A civilization may advance to a point where they need to use energy sources. Best energy source maybe geothermal vents and underwater volcanoes. this could provide energy for future technological development.
Now if your "merfolk" could harness this, they ...
Cegfault took the most obvious material: stone. Let me offer a couple more.
Houses could be grown through coral polyp husbandry. Officially, the material is calcium carbonate. Houses of nearly any size could be made through this method, though is would be slow. However, with a bit of engineering, walls, floors, and roofs can be built this way. ...
Depending on how deep you go, Nitrogen is not your only problem. Once you get below about 60m of depth, oxygen toxicity also kicks in and CO2 poisoning is also something divers really have to worry about. The way they deal with all of this is special air mixes that contain percentages of inert gas to reduce the amount of oxygen and nitrogen they breathe in, ...
Okay, I'm going to go there.
It's going to happen. You know it, I know it. Mermaids know it.
Professional mermaids say 'merverts' are making their lives a hassle
Can't have a believable economy without the oldest profession.
Territoriality. They would want surface dwellers to stop doing certain things, or start doing certain other things.
Stop dumping that filth in the river! Whew! It really stinks. On the other hand, if you dump your raw sewage in this other river, it will nicely fertilize the commercial kelp beds.
Stop using drag nets to fish in this area. That's our garden ...
Assuming the mer-people can breathe through water, and mirroring Native American style homes, then it would be practical to form homes out of sedimentary rocks at the bottom of the sea. These would likely resemble the homes of cliff dwellers or adobe-style homes.
Most rock at the bottom of the sea is sedimentary; this makes it dense and hard. A creature ...
Sadly, it's physically impossible to build an endotherm that breaths water nomatter how hard you try.
In best case scenario, oxygen saturation of water reaches about 10 milligrams per liter. Consider the oxidation of beryllium metal in oxygen, one of the most efficient heat generation method possible with a standard enthalpy of formation of -599 kJ/mol.
Sound is the most effective way to communicate across a vast expanse
of ocean – travelling at a speed of five times greater under water
than in the air - so it’s not surprising then to discover that Blue
Whales have evolved the ability to communicate with sound across the
water. ... Due to their solitary lifestyles, Blue Whales have ...
They might have a specialized organ much like porpoises do. It could have bandwidth enough to communicate complex states of mind, to a point where it could be considered a limited telepathy (something of the kind happens with the Tines in Vernor Vinge's A Fire upon the Deep).
Human-like body language has the problem of interfering with swimming. You ...
How they might protect themselves depends on how far they are from land. Especially from land close by with high cliffs and the like.
500 meters is not at all deep for the ocean. So these balloon cities are either near a landmass or over an ocean mountain.
If the cities are too distant from land for an attack with mechanical devices to reach them, their ...
The reason we rely on our hands and faces so much when communicating is because they are very good for it. The hands and face have lots of muscles allowing for very precise control. This also means our brains have very good ability to control hands and face. And ability to understand movements of hands and face by others. The two are linked due to mirror ...