i have brought a number of birds from auction and on the whole have been ok with them.
however, i had problems with a local breeder who used to post here and set up her own set up which then spread mycro to my flock and had to cull them all. i quaranteened for 2 weeks and was still unlucky.
i think you need to be aware of things thats might crop up and try to be as vigilant as you can.
i've got a speck sussex cockerel from melton who is a proper gent. he must be around 3/4 years old minimum. but on the day i put my hand flat against the cage wire of a number of roosters and a few of them went for it. so i knew not to go for them. 3 of my 5 gold brahmas were from there as well and they are thriving (bar one who was caught by mr fox).
and just last week i picked up 2 white brahmas (mean men were talking of cooking them when they got home and don't know what came over me - bid for them and got them for a tenner!). one was a little sniffle-y but is ok now and well settled.
i make sure i keep the birds seperate for 3 weeks now and also check them for mites as soon as i win the bidding. you can alway go to the admin office and complain about the health of the animals before they leave the premises. i would argue the point that you are not allowed to handle or inspect the birds before hand and therefore the seller should be held accountable. however, if the birds are visably ill then its your own fault for bidding in the first place.
all in all - if you're going to be unlucky, it doesn't matter where the birds come from. just make sure you check everything you possibly can before buying/bidding...
eyes bright, lids opening and closing normally, healthy wattle and comb colour, no discharge from nose, clean and smooth legs, shiny feathers, clean vents and birds not huddled at back of cage. then after bidding check for lice and mites and if there are any concerns go to the office and complain straight away. its likely that the seller will still be around as they will need to remove any unsold birds.
it also helps to get there early and talk to the auctioneer -who might stand with you and let you check a bird if you're lucky.
hope that helps. sorry for the long post and best of luck with whichever option you choose