I'm looking to get rid of my singles and have some trades that I'm looking to sell or trade (preferred) I've been working on my list: http://lynnamacher.net/4trade.html
I just recently discovered half.com. I got 3 30 days of night trades for 9 dollars. I also got the first hardcover of Y: The Last man for $10. There are a lot of books for 0.75 cents. If you find someones store with a bunch of stuff, you can save a good bit on shipping.
If you're in the city, a comic book warehouse next to CostCo will open its doors on Free Comic Book Day weekend. http://www.kochcomics.com/ Selling lots of random single issues for a dollar.
Also, take a look at gohastings.com when they have the next % off on used books. Condition may vary, but I have gotten some great deals there lately such as the first 3 Y the last man and first 3 walking dead tpbs for less than $5 each.