Epitaph: With a few exceptions, the companies do ban the sale of virtual goods. They just can't effectively enforce it.
Not too long ago eBay started warning users that making more than (IIRC) 200€/a means you have to pay taxes on that money. Two accounts going for 120€ each would already put you into that bracket. Of course the biggest problem are the farmers who usually operate entire companies to gather stuff all around the clock and sell it. Would be funny if those big gold exchanges were shut down because of tax fraud...