A rental property is sold subject to the terms of the lease. The terms of the lease cannot be changed until the lease term has been completed, unless all parties to the lease agree in writing to amend the existing agreement. If there is no written lease agreement, then the tenant is on a month-to-month rental agreement and a 20 day notice to vacate must be served (Washington State) prior to the first of the next lease period(month).
Be very careful, an occupant may be a tenant even if the occupant does not have a written agreement and isn’t paying rent. Don’t assume that an occupant can be quickly or easily removed.