People often think that they will forego the expense of an attoney even when they are spending thousands of dollars on a site. This can be a big mistake.
When you go to purchase a site, make sure the attorney is involved. You should have an attorney that specializes in intellectual property and that understands Google link strategies. In this way, you can be protected. If anyone knows of some good attorneys like, this please post.
Anyway, the attorney will want to have the seller sign a non compete agreement, have a contract that includes all the backlinks, and payments tied to performance. The contract should be very specific. Nothing should be vague.
I am not an attorney, but it is wise to get one, otherwise your chances of having problems is much greater.