I always found buying used a bit risky, but nowadays you can find gems at a reasonable price on the used market. I am always trying to minimizing the risks by contacting the seller in real life (telephone, email or face to face)
Here are a few sites where you can find interesting stuff:
a very interesting source but probably also one of the most dangerous ones (lots of fakes, and just too many people just trying to steal the money, but also very interesting stuff and honest people) - up to you to decide
one of the best sources for the US. They have the reputation of being less reliable when selling in Canada or overseas (difficulties and cost of return policy)
Specific for Belgium and the Netherlands (Dutch speaking people)
http://www.2dehands.nl
Interesting second hand site for dutch speaking people (Netherlands and Belgium)
http://www.2dehands.be
Similar as the previous one, specifically for Belgium
Specific for France (french speaking)