I go to used book sales about once or twice a year. Recently, I bought some books that I've been meaning to read for a while. However, when I started reading them, I realized that about a quarter of the books I purchased weren't in great shape -- no missing pages thankfully -- but pages that were about to fall out, a broken spine, brittle paper, etc. I'm not really sure how long they'll hold up for.
I obviously should have checked the books before I purchased them. To be honest, they were really cheap, so I was more focused on getting as many books as I could for fifteen dollars (my usual budget) than I was on actually making sure that the books were in good condition.
But for future reference, what should I look out for when purchasing used/old books?