Do not accept checks for over the price.. this is the scam, the check (or money order) is always sent for well over the price, then the sender expects you to send the excess to some obscure other person. Meanwhile, the check bounces better then a super ball in a concrete building and you get stuck for the full amount not to mention you can get into trouble for passing a bad check.
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/