Usually you can have a good look round first before bidding starts, if you see what you want, have a good look at them, if they have an age on the cage good if they don't, they may say at the beginning of the lot, presumably you will want POL or similar, have a look at the condition of them, if you dont like what you see don't buy,(heart can rule the head, I know!!) can you tell the difference between a young hen and an old one? Just asking cos it would be in your interest if you can.
When bidding begins, as you say it's very fast, make sure you can be seen by the auctioneer if you bid, in my experience bidding will be brisk on something like Light Sussex, also have a price in your head that you want to go to, if you are not confident let someone else start the bidding and then jump in, I don't know where abouts you are but round here Light Sussex at the weekend sales would go for around £20 per bird even at auction! Thats another point they usually sell per bird, and chicks are usually sold per lot as are eggs.
If you get there early enough you may be able to get a catalogue if they have one and you will need to register for a number.
If you buy something, quarantine it for 2 weeks and I always dust with louse powder etc etc
Cant think of anything else but I am sure there is loads, hope that helps
and good luck