It's something that I've mentioned briefly before and something I will delve into deeper in the future but please take a look at this piece written by Michael Fremer on the excellent Analogue Planet site.
A Cautionary Tale
http://www.analogplanet.com/content/buying-used-records-cautionary-tale
Remember, if you a paying a significant amount for a record, do your research and know what it is you are paying your hard earned cash for.
Happy digging & stay safe!