where is the rear oil seal located?

The rear oil seal is located at the rear of the crankshaft, otherwise known as the rear main seal. The transmission will need to be removed to replace this seal.

Before tearing into the engine, make sure your PCV valve is operating properly, a replacement is cheap. It's located on top of the valve cover. ALso, make sure the hose attached to it is not kinked. If the PCV isn't bleeding off pressure inside the engine , the pressure can force oil out of seals which are not worn out.

Can this be done the same day it is sold to a new owner the same day? And never tell the new owner?