AC pretty much summed it up! If anything else go for the people watching!
I worked for a few horse auctions back when I was young. I was paid to ride ride horses through. Also the owner would pick up horses, I would sneak rides on them and next month I would ride them through again. I also would buy horses and do the same but ride them at a different sale.
Things to think about... I would go to the auction a couple times before you think about buying a horse. Watch who brings the horses in. Horse Traders(the sneaky ones) usually have a pattern. They hit the same sales and if they got a bad rep. They will pay one of the auction kids to ride his horses through or find some young kid who wants to a horse trainer to do it, so he isn't seen with the horses and no one will touch it with a ten foot pole. Usually you see the same people every month and you learn who are the horse traders, the tack traders and the wanna bes that die out eventually who can't make a little money doing it.
Also pay attention to the ring men, at some of the less reputable sales, the ring men will call bids that aren't there to roll the price.
Pay attention to who your bidding against.
If you go to neighboring sales, pay attention to the horses, sometimes if a horse is really bad, you will see him at a different sale the next week.
Doping, if you see horses that are maybe a little lethargic, their penis is dropped, he is stretched out but not peeing..pass. Not sure what drugs, if any they use now, but back then Ace was popular and accessible and if giving just a little too much had that effect.
Not all horse sales have crappy horses, but you really have to pick through them. In my opinion you have to have a keen eye for horse flesh to get a super buy.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.