I could see the market now.... one player buys all of one item, then sells it at a higher price later. So 50% will soon be 55% then 60% and so on. I should know, I have done that on multiple occasions in other videogames....
Edit Because of poll:
If there is going to be a player market, there should be a type of "tax" on the player's items. For example, say a player sells an item for 100 credits. They will only get 80 credits back after a player buys the item because there is a 20% tax on the item they put it into the market.
Also, After so many credits, the "tax" could increase by 5% ( or some other random number.) So, if one player sells an item at 100 credits, there will be a 20% tax on the item. A different player sells the same item at 200 credits, and the item will now have a tax of 25% on it.
That is what I think should be done if a market is added to Super Mechs.