Often times, they are ready to leave anyway, or would leave with an incentive, such as 1/2 month rent.
I hope it works out well for you.
In 2009 Obama passed a BILL protecting Tenants, primarily because of the Housing Crisis.
You should double check me, but, in essence;
If there is a Tenant and they have a LEASE, that lease must be honored by the new owners.
If they do not have a lease, they are allowed to stay as much as 90 days, and I think they may receive some Moving Money from our Gov.
Good luck and may God bless