It's not too hard to write a program to monitor sites with stuff for sale, to scrape the data, and automatically generate the inquiry.
If the scammer gets a bite, then you get a phony cashier's check in payment. You ship the goods, and later on find out that the check is phony. Your stuff is gone, and the bank takes the phony payment back out of your account.
Sometimes there will be a song and dance about being in the military, stationed outside the country, and sending a representative to pay you and pick up the goods.
I put a $575 ham transceiver on KSL.com, and within minutes had three inquiries, all with red flags. So later I added to my ad: I already have six offers to exchange this item for a phony cashier's check, and I don't need another one. That reduced the flow. Finally, I had to add: Face to face cash transaction only. That did the trick. Now a ham about 60 miles north of here is on the air with that transceiver.