Oak Tree is one of the few surviving used book stores in Tulsa. Used books are $3.50, and they have a great selection, all very well organized.
They ONLY take books that are in good condition, and are relatively recent (within the last 10-20 years). So you can buy them cheap, or sell your own for $0.50 each back to them. But be warned - they're kind of picky about what they'll buy.