I sell books on Amazon and on Half.com. Most common books have hit the minimum listing price though. The only money to be made selling books online is in volume of sales, where you can make around a buck on the shipping reimbursement, or finding the rare, in-demand stuff.
I buy a lot of paperbacks to read at garage sales and thrift stores. I, or someone in my family, will have a yard sale each summer, and I sell them there as much as I can, usually three for a dollar or something. When they start to pile up, I just donate them back to the Goodwill. Although lately I've taken to supporting more local stores, that put their money back into helping my community directly.
I also trade books to a local used book shop, they give me decent enough credit that most books I want to read are around $1 actual cost for me, assuming I bought the traded in books for a quarter. It's a nice way to pick up stuff that would still cost me $4 if I bought it from an Amazon seller, but you're reliant on their selection.