I love auctions, and have bought a lot of equipment from auctions, including tractors. Make sure you check out the equipment very well. If you're not very mechanical, bring someone who is, with you, if you can. Go to the viewing before the auction and figure out what the piece of equipment is worth to you. Have that number in mind and DO NOT bid above that number. It's really easy to get caught up in the bidding and think to yourself, "Well, just one more bid". One bid can become a bunch of bids and you end up paying too much. I've never had a problem with anything that I've bought at auctions, and I usually get it for a better price than I could have buying it privately. Hopefully, you have several options to bid on, if the auction is big enough.
Good luck, and let us know how you do.