The obvious thing to do is a search on Google Image, and see for yourself what a PS2 controller looks like. Unforunately, they're very easy to duplicate - so it wouldn't really matter how much they're selling it for. The best thing you can probably try to do is bargain for as low as possible. Also, see if you're allowed to field-test the controller either on the spot or take it home for a day and decide if its good enough.
Even if it decides to crap out a few months later, getting another one or a replacement is entirely possible. Heck, make the guys who you brought it from offer it for free! You're the customer, so you deserve some kind of accommodation~
