Randy Farmer has just posted his upcoming State of Play 2 conference paper, called "KidTrade: A Design for an eBay-resistant Virtual Economy." Dan Hunter at Terra Nova sums it up nicely.
He suggests that there are virtual economy models that are resistant to eBaying, and which (talking to game devs) "may be suited better to your property, especially if externalizing virtual object markets will be harmful to the health and/or profitability of your product." He outlines one instantiation of such a model for an "eBay-resistant economy, designed for children to trade scarce virtual objects without fear of being cheated by smarter, craftier (adult) traders who are generating a lifestyle-supporting income from eBay-ing the kids' poor trades."
(via Terra Nova)