It would depend on the auction/auctioneer that you will actually be selling at.
Usually a reserve amount is allowed. You set it with the auctioneer before the sale. The auctioneer can also probably tell you if the reserve is reasonable (if you want $50,000 for a $10,000 tractor then it is just a waste of time). If the tractor does not sell, there will be a "buy back" fee that you must pay.
You do not need to set a reserve. If you don't, realize that the sale is final. If the tractor sells for $15 you can't change your mind.
When you decide to sell, I would recommend getting in touch with the auctioneer early. There is usually an advertising deadline about one month ahead of the actual auction. This will ensure your tractor is listed on the sale bill and can bring in buyers (more money for you).
Make sure it will run (gas, battery, etc). Nothing drops the price like a tractor that won't start. Make sure somebody knows how to start it (the auctioneer, or you standing there during the actual sale). This is especially true if it is something a bit different (pony-start diesel, hand start, etc).