This is something that has been implemented in Minecraft as a server mod. It works well there because someone can go and collect things to sell and if they sell for more than the fee, prpfit is made. This system would only work if it were possible to find random drops while playing in game.
There is also the issue of finding low value items that cost less than 200TC or 500TC. Auctions won't be as successful, and the auctioneer loses money if he/she decides to sell the low value item. People will come to this realization instantly, and it will lead up to a buildup of low value items in the person's inventory if they leave it alone for long enough.