I think the easiest and most necessary part here is the rule that the more you sell an item, the lower the selling price of that item. Maybe it makes sense to make a property in an object that introduces a coefficient to this formula. Say, selling trophy AK-47s after some time will not be so profitable, but rare and valuable mind probes will always be in price.
All this might be quite good for correcting a problem that I often hear about in XCF - in lategame the economy breaks down and the value of money is no longer there. Of course, this needs to be fixed in the mod itself, and only modifying the engine for this is not the clearest way. In addition, we will have to store in a save file a quantity for each sold item, and read this quantity every time the sale price is calculated.
By the way, if you store this data in a save file - we could do this - by buying an item we reduce the value in the counter, and by selling - we increase it. We can store negative numbers, right? Then one integer will be used to modify the price of buying and selling, and selling a lot of items, and then buying them, you could correct the dynamics.
But still, these rather massive changes in the mechanisms of the game, will only implement the simplest mechanics, price changes will be a constant parameter and there will be no relaxation of supply and demand, this is a separate mechanics.
I can't say that this is a priority thing, although in my opinion it would help solve some problems. If we define the design, I can work on the implementation, it seems not to hard - we need advance of the methods of getting the price and incrementing of new records on items in save after approving buy/sell (and output them in save).