You can't buy/sell aircraft that are currently on a lease. However, if the aircraft is returned, the owner of the aircraft could then sell it on in a private sale to the former leaser.
One of the fairly common suggestions for the game is a lease to own system, whereby you lease an aircraft for a certain period of time and then can automatically purchase/receive it at the conclusion of the lease. This isn't currently implemented, so if you want a lease-to-own scheme, you will need to trust the person you are leasing from/to to actually sell/purchase the aircraft.
