I buy a lot of stuff on ebay; I've gotten most of my camera stuff from there and mobile phones. I saved a hundred quid or more when I bought my current phone, a Nokia N8, on there. Someone had bought one, decided they didn't need it and stuck it straight on ebay.
I always check their feedback rating (must be at least 99%, and at least 100 ratings) and then, perhaps more importantly, look at the photo of the item. If I see a photo taken of the phone, box and accessories all laid out neatly on a surface, then I know it's going to be absolutely fine. If it's a crooked or blurred photo of just the phone or just a standard promotional pic, then I steer clear. I've never gone wrong following that guide.