just be careful, be very familiar with the breeds you are looking for and then look at the birds for the expected characteristics. especially if you are looking for breeding stock. leg color, size, type, all the specifics for your breed.
Ask the seller what they have, if you tell them "I am looking for dodo birds", some will have dodo birds for sale, even if they don't!
there are a LOT of honest folks and great folks at the swaps, just use your common sense. if its too good to be true, it probably is.
And get there early! The best birds will sell early
M