In DC tenants have very strong rights and can make life difficult in general when their home goes up for sale. Buying a home that is tenant occupied, you can evict them only if you plan to live in the house on the following basis: the tenant must be given 90 days' notice, in English and Spanish. The person claiming personal use and occupancy cannot rent out the property for a full year following possession.
Here's a great website that explains tenant's rights in selling property in DC:
Kangal Real Estate
Is your settlement scheduled for after the tenants lease expires and they have moved out? That would be prudent.
Check with your settlement company. In a recent sale that I worked on our settlement attorney required that the tenant sign an affidavit stating that he had no intention to purchase.
Good luck with your new home.
Long & Foster Realtors