I bought nearly all my books used through Amazon, with the occasional exception when a book was cheaper new through Amazon or AOTA. PDFs were also handy, but I preferred having hard copies. I'm planning on going into phys dys, so I've kept related books. The others I've sold through Amazon's trade-in program, which has worked out well for me. I have PDFs of most of the books I've sold back, so I'll have them for future reference and the NBCOT.