Interesting quote from Massively‘s GDC coverage about how to design for Real Money Trading:
Bethke imparts some advice on how the common mistakes can be fixed in terms of balancing. You can charge for things that are defensive in nature and the less skilled players with more money can buy them, while offensive items remain free of charge. The more skillful, more time-rich players won’t resent the advantages the other type of players has, as it won’t really stop them from doing what they want. Some companies have buyback programs for overpowered items, but this can set a bad precedent.
Lots of other interesting perspectives came from the panel – worth reading the whole thing.