To the honoured Ebyssybl Mollassand,
Unfortunately my agency will be unable to assist until you are willing to take a holistic solution to this problem.
Your continued insistence on using freelance agents to address a problem that should be handled by permanently retained specialist military units has led to a large accumulation of unsavoury elements within your kingdom. These merchants, whose behaviour would be considered criminal by any other standards, are tolerated because they are willing to do business with the "adventurers" upon whom you depend for internal security. Should your kingdom be attacked by an external power these adventurers, who it must be said have have no loyalty to the kingdom or indeed to anyone but themselves, could well end up fighting as mercenaries for our enemies. These freelance adventurers should be absorbed into your standing army and directed to the regions where they can do the most good rather than where they can gain the most profit. It is essential that the security of the nation is not put at risk any further.
A healthy pay and pension scheme, along with bonuses and rewards for securing strategically critical areas and recovering valuable equipment, out of which they should certainly be able to take their pick, would easily outweigh the short term benefits and risks of the solo adventure.
Your faithful servant,
Guild of Anachronistic Trade Specialists
On the kingdom
It's clear from your description of adventurers that your kingdom has a problem. A country only exists so far as its own military, logistics and infrastructure controls the territory they claim. Especially in a more warlike age, you can sit there ranting that it's your land by right, but the only right is the right of the sword and if your army can't control that land it's not your kingdom. The same applies whether it's hostile armies, mad mages or troggs. Your territory, your army, your problem. What you can't control reverts to the wilds, the badlands, no-mans land. Once you have to start getting external agents in to control the local pests, people are going to start sniffing around what else you can't control, like your borders perhaps.
Adventurers should be out adventuring, this is not something that happens within the borders of your country (maybe within the borders of someone else's country though), what happens within the borders of your country is policing and pest control. If mad mages are a common regional pest then your army must be equipped to deal with them. Adventurers are basically criminals with a license, privateers on land if you like. They rob, the kill, they steal, they trespass, they cause ecological disasters. They upset gods and men alike wherever they go. They don't care about local predator prey balances, they don't care that dragons are endangered, that rocs are almost extinct, they don't care about local economies, they don't care how much of a mess they leave behind them on the road littered with the corpses of the people/beasts/things they meet. All they care about is glory, beer, money in approximately that order.
To crush your enemies, see them driven before you, and hear the lamentation of their women.
This rather casual attitude of roaming adventurers to the wellbeing and economic balance of a region is a good reason not to allow them into the area, regardless of short term benefits they may apparently bring (apart of course, from the glorious sons of our nation returned home to spread their wealth).
On merchants turning into villains
Your merchants are villains, to be specific, they're fences. They buy goods of often highly questionable origin from the "adventurers" and sell them back onto the open market. They don't ask questions, they launder salvaged goods, stolen goods, whatever. Some master craftsman somewhere made this stuff, it certainly wasn't the adventurer selling it, it's equally unlikely the adventurer paid for it. Adventurers being adventurers couldn't really care less what the merchants do with the items sold to them, all they care about is getting the best price (and where the tavern/brothel is).
However there is a standard issue solution when it comes to merchants and other civilians attempting to use high end combat equipment:
A good sword in the hands of someone who doesn't know how to use it will soon find its way into the hands of someone who does.
(I've never been able to trace this quote)
Should this be a repeat problem rather than one quickly solved with an arrow to the knee, then a royal monopoly on purchasing anything that comes out of the labyrinth should be easy to establish.
A town that springs up in a place like this could at best be described as a frontier town. Its population is going to be the people who know that the way to profit from a gold rush is to sell picks and shovels, it's going to be thieves, prostitutes and innkeepers. The borderline mass murdering, grave robbing, temple looting, "adventurers" who pass through are not going to be people you want in your country. The best thing you could do with a town like Puzzleville on your patch is to kill it with fire. Then salt the ground and put in a permanent garrison to stop people accessing the labyrinth. You do not want the people who gravitate to a place like this in your country.
If this labyrinth has suddenly cropped up in the heart of your territory and you're looking for a way to exploit it then you have to take a different approach. The town should be a military town, the entrance to the labyrinth tightly controlled. Who knows what could come out or try to get in.
The garrison should hold a full kit inspection for anyone going in or out. Nobody is allowed in unless they are adequately equipped and experienced, all persons coming out again must submit loot they haven't chosen to use for inspection and purchase, at a good price, by the garrison quartermaster. The garrison itself should be equipped from this source. In return the adventurers passing through should be allowed access to the garrison facilities, catering, accommodation, blacksmiths for armour repair etc. Only the most mundane and mildly magical toys should be allowed onto the open market.