It’s quite simple, really.
If the deal involves sending money back to the person who mails you a check then it is a scam.
There is no legitimate reason why anyone would ever ask you to do that.
“There is no legitimate reason why anyone would ever ask you to do that.”
It’s disturbing how often this scam works, it’s also used constantly on people selling used items. It relies on the fact that banks make funds available immediately after checks are deposited without confirming that they are legit. (Somehow my credit card payments can be instantly deducted from my account but cashiers checks take a week to be verified.) The scammers will even tell their marks to “wait until the check clears the next day so you know it’s good” knowing full well that the funds will be available regardless.
I was exchanging emails about this problem with the Nigerian barrister who is helping me obtain funds abandoned by a deceased cocoa plantation owner.
It is fun to force them to spend the money and then tell them that you are turning the documents over to law enforcement for evidence.
I'm tempted to demand that they send you $50 in cash not to do so, but that would probably get you on the wrong side of the law.