Not all foreclosures sale will sell at a public auction. All homes will go to the auction block and if they do not sell they will sit on the books of the bank until they can sell the house. If the home doesn't sell then the bank is going to be desperate. No bank wants to own a home and their goal is to sell it right away. This puts you in a buyer's position and you have the advantage over the bank.
An auction foreclosures sale with HUD or with the county is through a sealed bidding process. When you purchase homes this way you need to be pre qualified or have the money on you in cash or check. This is a bidding process and the highest bidder wins the bidding and the home. This can be discouraging and frustrating because you don't know if you get the home until the end.
When you buy a home through a foreclosures sale you often don't have the opportunity to inspect the home for damages or have the ability to learn the condition of the home prior to making a purchase. The problem is that if you are buying the foreclosure for an investment and there are too many damages then you may instantly be upside down if the damages are extensive. This happens often with investors that purchase homes through an auction.
When you wait for homes to go to the bank after the foreclosures sale you are more likely to get a better price and have the ability to negotiate a little more than you would before the auction. A bank will in most cases accept 20% off the top of the price in an offer. Always be firm and willing to negotiate with the bank. They are willing to negotiate but they want to get as much as they think you might have to buy the home for.