The problem with used vinyl is exactly that you don't know how well it has been cared for. A visual inspection of the record itself, and its cover / artwork can give you some comfort, but you really don't know until you get it home and try it out. I suspect most shops nowadays would let you try a used record in-store, using some set-up of theirs (and headphones), but how do you know that their gear is set up properly?
I believe I've cared for my original-bought records properly, and like rammis' they sound as good as when new, but then again I had some roommates in res at university who might not have when they helped themselves and spun them when I was out (at least on my properly set-up turntable.)
I don't buy vinyl any more, but if I did it would almost certainly be new only. Just about any recording that I want can be found in some digital format. (Example: I've recently used Spotify Premium to acquire a few dozens of albums worth of content that I'd been looking for, in some cases, for years.)
Jeff