I've been to auctions as a spectator.
I can only comment on the ones i have been to but..........
All the "lots" are clearly described by the Auctioneer i.e. cockerals, pullets, laying hens, POL, breeds etc
Some of the more reputable sellers attatch a label to the cage giving more info on any vaccinations, worming, hatch dates etc and even provide their telephone number for any follow up queries
If you are planning on buying, get there early and give the birds you are interested in a good look over, sometimes you wont be able to handle them but you can clearly see them through the cages. Take a good look at the feet and legs, any feather loss, eyes, etc.
There should be someone there in an official capacity (usually someone who is booking the birds in from the sellers), ask them for info on reputable sellers rather then people loitering about as i maybe those people loitering about that are selling them. Someone tried to steer me (and i wasnt even buying) in the direction of some alledged POL birds but they were in a shocking state and i told her so and pointed out why i thought they were in such a state. I think she thought i was a greenhorn and wouldnt know better.
As for the prices, have an idea of what you are prepared to pay. If there is a "lot" that seems a little too cheap to be true, there is probably a reason why.
If you are planning on buying, make sure you have a suitable means of getting them home i.e. a box with ventilation, cage etc.
They also auction hatched chicks etc and they are usually labelled "as hatched" which does mean you dont know the sex.
Having said all that, they are good places to pick up feed, accesories etc at a considerably lower cost to breeders.