1. eBAY. This is a great source for relatively cheap books, DVDs, clothes, home decorations, and other cultural driftwood. The only problem with it is that it lends itself to overspending, because it's basically the world's hugest discout shopping mall, where 50% of the items are awesome and the other 50% are funny in their lameness. So it can become a pretty massive money-suck if you don't go in with a list and stick to it. I mean, shit, last night, I was just trying to replace my old Nexus books, and I came dangerously close to bidding $500 on a piece of Steve Rude original art. I don't have the willpower for eBay and should probably avoid it in deference to ABE Books or some other site that doesn't give me the option to blow most of my paycheck on a drawing of a four-armed ape-man done by a man nicknamed "The Dude".
2. FREECYCLE. There are two problems with Freecycle, the outfit that basically functions as a huge internet version of the "Free" section of the want ads. First of all, like the "Free" section of the want ads, much of the stuff they offer is garbage, stuff that the owners would otherwise just toss in a dumpster -- otherwise, they'd be selling it on eBay. While occasional good stuff shows up, it's mostly junky kitchen appliances, outgrown kids' toys, and furniture too shabby to keep and too big to move. If people have actual worthwhile stuff they don't want, they tend to sell it. Second, unlike the "Free" section of the want ads, Freecycle makes you jump through many hoops in order to place yourself in the running for a paper bag full of fabric samples: I had to confirm my e-mail address three times, fill out a lengthy questionnaire, and provide a 200-word justification for why I should be allowed to browse the used coffeemaker entries before I got access to the San Antonio Freecycle site, and it hasn't exactly been worth it so far. And the ban on reposting requests means that you can ask for something once, and if you don't get it right away, you never will, because no one looking to give is gonna bother to wade through the hundreds of previous posts to see if there's a taker for what they could just as easily drive to Goodwill.
3. PAPERBACKSWAP.COM. This one, on the other hand -- which I learned about via my friend thaitea -- is pretty awesome. How it works, for the three of you who don't already know about it, is you post the ISBNs of books you want to get rid of. If anybody wants a book you're unloading, you get a message, and the site provides you with printable media mail labels with the requestor's name and address. You just wrap the book and send it to them, and get a "credit" good for one book of your choice from OTHER posters on the site. You can search for titles you want, make a wish list, and shore up your library with any number of titles you're looking to unload. I've already gotten requests for 3 of the 12 books I listed, and since you get 3 credits just for listing 9 books, that means that I can replace 6 of the books I left behind with relative ease. It's not entirely free (you have to pay for postage on books you send, which, at media mail rate, is $1.59), but it's still much cheaper than most used bookstores or even thrift stores, with an easier interface so you can find exactly what you want. Heartily recommended!