Suppose this is in CA, USA

The renter's rights should be clearly defined in the lease agreement. Anything not defined, you can ask your District Attoney's office for governing laws.
In a house selling/buying contract, the owner MUST disclose whether the unit is being rented out and how the rental will be settled. You can try to contact the buyer about this retal situation, Any rental dispute will hurt the house price. The house is not completely sold until it is recorded at the county government.