I received a text that they wanted to purchase what I had for sell, but would need to send me a cashiers check because they were out of town and would have their movers come pick up. I passed.
Sending fake checks through the mail is a crime so contact the US Postal Service Inspectors:
https://postalinspectors.uspis.gov/forms/MailFraudComplaint.aspx
https://www.usps.com/