I would say you have to buy very carefully. Tell you what I have done, spend a few bucks and take a car man (mechanic, garage owner) with you to the auction and you will save money in the long run.
Those boys can spot problems about ten times quicker than you or I can, and it is worth the cost of hiring one to go with you if you are pretty certain you are going to purchase a car that day. I have a car salesman who restores cars in his spare time, and he can look at the car and about tell you what it is worth and what you will have to do to bring it up to snuff. A good deal if you ask me.