(MONEY Magazine) – If you are in the market for a secondhand car, you may be tempted to do your browsing at an automobile auction. These events, once open only to car dealers buying wholesale, are attracting large numbers of individuals who pay, on average, 20% to 30% less than the retail value. But be forewarned: while there is always a chance that you will find a real plum, unless you take great care you may run a bigger risk of getting stuck with a lemon. Most public auctions are conducted by a handful of companies with branches in several cities. They mostly handle cars on consignment from rent-a-car companies, individual owners and banks and finance companies, which have repossessed the vehicles from delinquent borrowers. Cars are generally sold ''as is,'' meaning that you have no legal recourse if your purchase breaks down as you are driving it home. You will usually have to pay a fee or $50 or so to enter the bidding (a few companies will refund your money if you do not make a purchase). This entitles you to examine the cars for several hours ahead of time, although it is unlikely that you, or your mechanic, will be allowed to drive the car. To reduce the chance of getting a stolen car, match the car's registration card on file with the auctioneer with the stamped identification number, visible through the windshield. Before making a bid, determine the value of the car by consulting the National Automobile Dealers Association Official Used Car Guide, updated monthly, which is available at many public libraries and lending institutions. At most auctions you will be required to pay cash on the spot, although a few companies will accept checks or credit cards.