A listing fee would be a mixed bag. The root of the problem is endless supply and limited demand. There should be a daily or, perhaps, weekly amount of materials a player can gather. It is rather silly that I can chop at a tree endlessly for items. Resources need to exhaust themselves for each player. A cap of, say, 99 of each item per day or week would force market diversification by sellers and damage the ability of bots to gather endlessly. While it would hurt bulk sellers, such caps should stabilize the market supply. It creates scarcity similar to what markets have in reality.