Pratically, you cannot resell or even give away your purchased iTunes songs. You agreed away your right of first sale when you clicked through the EULA when you signed up for the iTunes account. It's the same reason why you can't resell a copy of AutoCAD if you no longer need it or want it. You don't have the right because that right was agreed away when clicking through the EULA to install the software. Same issue with Amazon MP3, HDTracks, or any of the other digital download purchase options. They all have a clause in the EULA that makes the music licensed and not sold. It sucks. Had you bought $800 worth of CDs you could resell them or even give them away to your local library or pass them out as party favors. But with $800 worth of iTunes you cannot.
It's a tricky issue legally. IANAL. Yadda yadda. I don't understand it all and there are lawyers who also don't understand it all and the implications. Here's a neat article highlighting some of the confusion that even lawyers can't quite figure out: iPods, First Sale, President Obama, and the Queen of England
And here's a brief news article about someone who tried to sell a single iTunes song on eBay back in 2003: eBay mutes iTunes song auction
He was eventually able to sell the song. He had to transfer the entire account to the purchaser. Had to change the associated credit card to a pre-paid card and then give the purchaser the login info for the entire account. You can google for more info about this particular sale and the complications.
I buy my music on CD so I retain my first sale rights. I can resell what I've got or give away a CD as a gift. I'm not going to invest thousands of dollars in a music collection that I cannot own (to the extent that one can own a CD or book).