I usually buy used games from Ebay UK - often fast shipping and the prices are MUCH lower than here in Sweden. I'm cautious though, and only buy from sellers with good reps.
Do NOT buy Hong Kong games. They *may* be genuine, but DLC won't work with them unless downloaded in the Hong Kong region. Found out that the hard way with Soul Calibur IV, which was LISTED as "UK seller, Item location: Canada" but that was just the location of a local office of "Ching Ding Dong Ltd." or something and the package was shipped from Hong Kong and naturally contained a Hong Kong release. Which looks genuine and works fine (except DLC doesn't work - wasted money), but I was still miffed because that sure is fraud.
Also "Factory sealed" should also always ring a warning bell - it may be sealed at a factory, but chances are high the "factory" is a pirate pressing plant. It's common to find anime releases with this description on Ebay - despite the fact that NO LEGITIMATE VERSION with "english subs" (btw, that ALWAYS means very, very, very bad engrish, if you are lucky) has ever been released.
So caution is advised!