I used Google Earth to track down the address given by one of these spammers. It was a vacant lot in Sydney, Australia!
I had another say that their courier would be in my area to pick up the painting, and I should give them the “change”. I suggested they just have the courier bring the payment. The chances of their courier “being in my area” were extremely remote. I am in the middle of nowhere.
Often the scam works by sending you over payment by check and having you return the overage electronically, the check the bounces and you are left with the bank fees and the money you “refunded”.
One of the clearest signs of this scam is that they do not have a specific piece in mind, only a price range that may not even match the posted prices you have for your works.