Here are a few more pointers:
Do your research ahead of time and know what to pay for what you want. Use Machinery Trader or sign up at Ritchie Brothers and use the auction results to compare prices. Learn how to frill into the results by age, auction location, etc.
If you can, attend a few live auctions prior to going to one with the intent on buying. It is a very fasxt paced environment, and a lot of buyers are experts at it. It can be very intimidating the first few times.
As mentioned, test run what you want to buy in advance. Look hard at it, and know what to look for in any given machine. I have bought a lot at auction, and have had winners and loosers.
If you can, find out where the machine came from, and any history you can. If it is from a total liquidation, then you could have anything from great condition to poor, but if it is a partial selloff, it may be likely to be the dog of the fleet. I know anything I have put in the auction I would not want a friend to buy.
If there are multiple pieces of the same thing going to sell, and the auctioneer gives choice, the best advice is to be the first buyer, and take the best machine of the group. Many times the best machine sells first, and the price climbs on the others, so the lesser machines sometimes bring more money than the best one. This only happens if there are more buyers for that machine than there are machines available. It takes experience to learn this.
The number one rule is do not get caught in a bidding war and end up paying way too much for a machine. That goes back to your research, and setting a price in advance. Sometimes if it is close, 1 bid over your max is OK, but no more.
Good luck