For those of you who are always looking for a cheap way to add to your collection, here's an idea for you.
I have a close friend who's selling a bunch of her YA books (many new, some used but still in excellent condition) on eBay. You can pick any 15 books you want from the list on the auction, which has 48 different titles to choose from. There's a good mix of both hardcover and paperback, as well as well-known and less-known authors, including books like Forget-Her-Nots, Invisible Things, Teenage Waistland, books by Beth Kephart, E. Lockheart, Meg Cabot, and more.
Right now (at about noon PST) it's going for just $4.24 plus shipping (for 15 books), so it's a super deal. I would bid on it myself, but I already own or have read most of the books she's got on there.
Anyway, check it out if you're interested. The auction ends tomorrow (3/17) so you'll want to jump on it while you can!