Depends on the auction usually. Some of the higher end auctions I've participated in required me to pay a $20-$100 "processing" fee along with provide my drivers license and a statement of funds.
The others just required my drivers license and had a provision that you must put 10% (or $500 in some cases) of the vehicle selling price down, sign a contract, and provide the full amount within 5 business days. If you missed their deadline for full payment there are LOTs of fee's / penalties and other badness.