If you never signed a written contract and did not have an oral agreement for a specific lease term, you may be able to demand possession of the house with 30 days' written notice. An oral agreement to sell a house is generally unenforceable. You should probably make sure that all communications with the tenants from this point forward are in writing. You may want to consult with a real estate attorney to determine exactly what your rights and liabilities are in this situation.
This answer is offered as a public service for general information only and should not be relied upon as legal advice.