Dave may have provided the answer. It's true that there has to be what's called "consideration."
Or you may be looking at teaser ads, where the house really is priced a lot higher. Or it might be an auction situation. However, many auctions--even those that start at $1--will have what's called a "reserve"--meaning that if some number isn't reached, then the house isn't sold.
If you can provide more details, we probably can provide a more complete answer.
Hope that helps.