While brainstorming today, we started talking about things ourWorld members perhaps could trade. Also, independent of this conversation one our members asked if they could gift their extra items to a friend (sadly, no, not yet.) The nail in the blog post coffin came with this article stating that Webkinz now allows a form of trading.
It seems so obvious, yet there are many, many pitfalls. "Gold Farmers", scammers, phishing, and general working of the system, are all valid concerns and the reason most young virtual worlds don't allow any sort of trading.
As far as I'm concerned, the community aspect of a virtual world is the interaction between it's inhabitants. For worlds where the inhabitants are primarily earners and consumers (i.e. almost all of them) the ability to treat belongings as real belongings and trade will be essential to success. For everyone else, there's always the 90% prediction.