I'm designing a medieval/renaissance world economy but I don't know what resources to consider when it comes to medium or long distance trading. I'm not asking for specific resources that are valuable, I'm asking what makes them valuable.
What are the characteristics of resources that get traded over long distances?
This question is based on the suggestion of dmm.
A medieval period long-distance trade good needs to be compact, resistant to decay, valuable at destination and refined product. The less a trade good has these four characteristics the less likely it will make a desirable long-distance trade good. In the medieval period, ships or caravans were the only ways to move trade good around. This comparatively small payload capacity to modern cargo ships has severe implications on what can be traded and how far it can go.
Compact
If a trader has the choice between two trade goods of equal value per unit, one takes up 0.1m^3 and takes up 1m^3, he will likely choose the former for his journey. He can sell one of the latter items or 10 of the former items. His profit margin will be much higher with the former trade good.
Resistant to Decay
A loaf of bread won't last more than a day before it becomes unfit to sell to someone who isn't starving. Fresh fruit shares the same spoilage attribute. On the other hand, cloth won't rot unless it's allowed to get wet and stay wet. Wood and iron also don't rot/rust if properly protected thus allowing them to be shipped much farther across the world.
Valuable at Destination
This seems obvious but is worth stating anyway. Something that may not be valuable to the exporter must be worth the price to the importer. If not, don't ship it.
Refined Product
Iron Ore vs Iron Bars - Iron ore is bulky and contains ~60% iron per unit of iron ore. While these percentages are high, refined iron ingots are 100% iron so they are 40% more valuable. This case is more a restatement of the compactness point. Aside from the bulkiness, a refined or manufactured product is generally more valuable than the sum of its ingredients.
Logs vs Lumber - A single log is a huge irregular lump of wood that requires special equipment to handle. While some wood importers will want the raw log, many others just want the lumber from the log. Thus, an exporter will be able to charge higher prices and improve shipping efficiency by turning the logs into lumber before shipping them. Also, lumber can be more compactly packaged.
For example purposes, let's take a look at Maya trade over long distances. I will shamelessly pull my information from here.
The Maya associated certain materials, such as jade, with regional fertility and others, like turquoise, with smoke. Both these materials were associated with their gods. I would suggest one aspect of a good's value could be derived from the local belief system, as we can see in the Maya.
Maya traders made use of cocoa beans (CHOCOLATE!), which were a high-value commodity back in the day. The bean could be used to make chocolate, but it was also a prized currency. I would suggest that one aspect of a good's value could be derived from physical benefit; in the case of foodstuff, goods that last longer will have higher value than those that must be eaten within days.
The Maya also conducted trade using Quetzal feathers, which were used for decoration. The feathers of the resplendent quetzal were rare and a critical part of Maya social structure. I would suggest that one aspect of a good's value could be derived from cultural benefit and rarity.
The linked site provides much more information about each of these trade goods and more than I have described here. The inquisitive individual is encouraged to continue their reading there.
Basically anything that is location dependent on its collection/manufacture.
Silk was huge because the Chinese had a monopoly on a fabric that had many wonderful qualities. It could hold fine details, was very strong, could even be used as armor.
Spices were big in Europe partly because they had bland food. But many spices were limited to where they could be grown, or how they could be grown.
So the more desirable and difficult to acquire an item, the more people are willing to pay for it, making it more valuable for it to travel longer distances.
You wouldn't pay to bring a potato from more than your local grocer (yes I know potatoes came from South America, how about a turnip?) because they are cheap and easy to grow (and weigh a lot!). You might go to a nearby city to buy a good tool (maybe a high quality sword). Maybe pay to bring something from the next door country because they have the best lute makers. So the easier or less desirable an item is the shorter the distance it will travel to its end destination. Horse manure tends to go from the barnyard out to the nearby field...
I think these have historically been the important points when it comes to the goods:
With sea-trade volume and "stacking efficiency" become more important in the weight/volume to value ratio because you're hull is only so large and its shape isn't going to change so even if you can spare another 500 tonnes of reserve buoyancy if you can't get that 10kg bolt of silk in the space between the bulkhead and the wine barrels it's staying right where it is.
Whatever and however you trade government tariffs, national border crossings (fees and border permeability), import license fees, cost and necessity of caravan guards, warehousing, sales venues, etc... are all going to play a role, you have to be able to get your goods to market and for a reasonable percentage.
In the olden days you have two ways of getting rich from trade.
Bulk staples such as grain, ingots, wool etc,. and luxury items, jewelry, weapons, fabrics.
Luxury items are best for small outfits, low volume, high profit. A luxury item can be anything that is not readily available in the home country and expensive. Assuming you're a small trader these are what you want to deal in if possible in order to really make profits. But they entail either high risk transporting or high initial outlay purchasing them for the same reason. For a robber gang they're expensive and portable, a shipload of grain is harder to hide, sell etc,. and would need a bigger robber band.
The pros of staples are you're not taking as much of a risk, they're staples, you will have buyers for the goods, they can be split into smaller portions and sold easily. The cons are you need to move them in bulk which takes quite an infrastructure to accomplish.
Intermediary traders would probably do a mixture, buy staples and speculate on luxury items. Thereby guarantee a profit from the staples for their hazardous journey, with the potential of making a lot more through sale of luxury items which take up much less room.
