I would wait until after Step 1 to sell any useful review books, but otherwise get rid of any basic science textbooks. I have never looked back at mine (the actual texts, I use my old review books a lot and will continue to until I take Step 1 this summer). Some people prefer to keep everything, but I don't have the room and the extra cash is nice, plus whoever buys from you gets a good deal as long as you set reasonable prices. Just my opinion!
Mary