Trader item categories are implemented? What? How? Where?
As for the original suggestion by the topic starter:
It's a great idea that will work only partially.
So demand increases supply, right? What would be the demand be biggest for? Things that are hard to farm.
What would people sell the most? Things that are easy to farm.
Where is the middle ground? It doesn't exist
Say, I want to buy a 14mm pistol and ammo for it often. Someone likes crafting, supply is full. I see it in the shop. I only need to pay for the item.
Now, my character can craft simple armours and likes killing bandits or slavers. I can offer to sell spears, (which nobody wants, so the shop is full) shotguns, (which nobody wants) leather armour, (which nobody wants) or fruit, healing powder, ropes. (all of which nobody wants)
How, in such a situation, where everything I have to offer is plentiful in encounters, can I get a 14mm pistol and ammo? The only solution I can think of is to spend lots of time crafting some stupid items, mining HQ ore, moving boxes and wasting more time on things I do not like.
Currently, I just check the store once or twice a day, see if he has 14mm. If yes I go back and bring gecko skins for sale. With the supply and demand, I would have next to no chance to get a 14mm pistol simply because everything I have is common, so players would not buy them from the trader, which means I would not be able to sell them to the trader.