When Selling
Selling the site does not mean selling also the AdSense™ account. Accounts are not transferable or resalable. First, you must cancel the account for the site you want to sell, as the accounts are nominal. The new owner will sign up for a new account for the site in his/her name.
If you sell the site that you used for signing up for AdSense™ you must first submit another site in order to keep your account.
When Buying
Check if the site was not banned from the program. You can find out through an email to AdSense™ team.
It is good to know, if a site is banned for illegal clicks, owner, content and domain are all banned, and even if the owner changes, they will not accept the site back into.