Ask alot of questions. Appearances also make an impression. The ones that really seem to care about their birds aren't the ones bringing crates covered in crap and jam packed with birds. You usually get what you pay for too. If you want something that actually looks like the breed you want, then go to a breeder. Educate yourself before you go too and don't let some of those old guys BS you.
For example.... you come across someone selling Rhode Island Reds. A true RI should be a deep mahogany color. There is a huge difference between an exhibition variety and the run of the mill production reds. Look for goopy eyes, raspy breathing, crusty vents, lice/mite infestations, etc. Sometimes you can get a cheap bird but the cost of treating it (and possibly the rest of your flock too) could break your wallet if you aren't carefull.
For example.... you come across someone selling Rhode Island Reds. A true RI should be a deep mahogany color. There is a huge difference between an exhibition variety and the run of the mill production reds. Look for goopy eyes, raspy breathing, crusty vents, lice/mite infestations, etc. Sometimes you can get a cheap bird but the cost of treating it (and possibly the rest of your flock too) could break your wallet if you aren't carefull.