All birds: somewhat clean, but not necessarily as clean as birds that are shown; NO parasites or evidence of them; bright clear eyes and alert attitude (healthy), a seller who takes time to answer your questions and teach you about the birds he or she has to offer--just realize that there are times at a show when someone is very busy doing last minute grooming or tending to their birds. If they are part of the show staff or breed club, they may have duties there.
Take cash (not all will accept checks) and be prepared to make a quick decision. Until money has changed hands a fair number of people do not believe a deal has been struck. I have known people to become upset when they said they wanted a bird, but made no effort to pay for it right away, and another buyer came and expressed interest and had money in hand.
assuming you are looking for exhibition or breeding birds,
Silkies
5 toes per foot
full crest
good beard (if bearded, or complete lack of beard if non-bearded)
heavy foot feathering
tightly held wings (not droopy)
dark eyes
dark comb & skin (the darker the better)
walnut comb
correct plumage for the variety
type: body overall round,
tail round, soft and smaller than body,
head/crest round and proportionately about the same amount smaller than the tail as the tail is than the body.