Just bought retail center and one tenant refuses to pay me and continues to pay old seller.

[Alexandra R.]

I just bought a 4 unit retail center. One of my tenants refuses to believe I bought the building and is still paying the landlord that sold me the property. I have given him the assignment of rents to show. He obviously doesn't understand. What are my options? I'm about to send a notice to vacate. Does anyone have any advice?

Also The seller that sold me the property keeps telling me they owe back rent and refuses to write me a check.