Unless the place you’re getting them from really doesn’t take care of them, or they buy products from people without checking to see if the discs are badly scratched up, then there’s no problem. I’ve bought a lot of used games, and seldom had any problems. Only once I got a pretty badly scratched disc, but otherwise I’ve been happy with all my used game purchases.
As @mowens mentions though, be on the lookout for games that come with special codes and stuff. For used games, you’ll likely not have the free code. (extra levels, weapons, costumes, DLC’s for free, wtv)