Selling "Used" MP3's

Can I legally sell "used" MP3's? Or, more generally, can I resell the digital goods that I buy online?

Could I create a marketplace for used digital goods? It would facilitate the buying and the selling and "ensure" (to the greatest extent possible) that the seller's copies are destroyed.

I read about a company called Bopaboo (used MP3's) but I'm not quite sure what happened to them.

I think a legal discussion might involve the "first sale doctrine".