I know this topic is due to be closed but would like to add a little.
Auction/user shops etc can be awful for cheating/boosting.
However there's many things that can be implemented alongside this idea to prevent this.
Item Levels
Min level for using AH
Charge/Fee system which would scale to item worth/stats. So a very powerful weapon could be listed at 1 coin to buy but with the associated fee for item value, it might cost 1,000,001 to purchase.
Another suggestion which i haven't seen before (A demeiz original ;D ):
When an item is placed into the Auction House/Shop an invisible tag is placed on it, Which indicates originating user, changes, etc. But also applies a Min Level Flag equal to that of the selling user;
I.e a level 400 places an item up for sale. Regardless of the items stats it cannot be purchased by anyone lower than level 400. (or maybe a +/- value as well for example 25, so it'd be 375-425 random? Or maybe just at 375)
This would mean that players are trading with similar levels. Items stay within level groups. And boosting is made much much harder.
Just my thoughts.