You either have to buy from someone here, Ebay, or from a secondhand kit dealer.
The dealer will probably be the most expensive as they have to make a living from this, but you will probably get a guarantee for 3 months or something.
A photo forum is probably going to be the next safest as you would expect everyone to have an interest in photography and you can check out the sellers previous posts to try and get a feel for things.
Ebay - who knows about the product? Generally no guarantee, but unlike the two other situations where the price is disctated by the vendor, at auction its dictated by the buyer. Ebay is a bit of a game and when you understand the rules, you can use this to best effect, if your a buyer or a seller.