Go as a spectator first and see what happens. If you don't know much about cars, take someone with you who does. Decide the maximum you can afford and stick to it. The entry form attached to the windscreen will give you an idea of the car's history.
Auctions are probably the riskiest way of buying a used car.
Your usual legal rights may not apply if the seller issues a disclaimer, such as the term 'sold as seen', which excludes all or some of those rights. Read the auctioneer's conditions of business carefully to check whether this is the case.