George Hotelling documents his successful attempt to resell a song bought on the iTunes Music Store. He acknowledges the difficulty of the task and found out that you cannot transfer a song but your entire iTMS account, which means you have to sell your entire music collection (although you can open several accounts and split your collection between them).
There is a significant used-CD market, why wouldn't we see an electronic equivalent? This is clearly an opportunity for Apple to include a flexible transfer function between accounts, which would further differentiate it from competitors. It seems easy from a technical standpoint, most of the work is on the legal side, as one objection to reselling an electronic song is that you are making a copy of the good, something that the first sale doctrine did not address. But a well designed transfer function of DRM rights would prevent illegal copies, as DRM already controls what you can do with a file.