Depends on the lease agreement. If there's something that says he can't sell the property, there you go. Even absent a promise not to sell in the lease agreement, the new owners of the property would have to honor the previous lease agreement unless there's a clause saying the agreement is void upon sale of the property.
You don't .. There is no guarantee that the property will not change hands. Whether you are paying rent on the land on a month to month or have signed a lease for the lot .. there is always a risk that someday, the arrangement will change.
When you purchase a mobile home, a park model or even a building on leased land .. you will always run the risk of having to move it someday.