Scams when selling on Craigslist or ebay

I’m wondering something about responses to adverts on these sites and many others.

The response is polite and wants first to know whether the item is still available. I respond, “Yes, still available”. The next contact says he likes the price and condition. Wants my name and address to send a check. I could wait until the check clears. Fine. Next contact he wants to send extra bucks in the check to pay the shipper after I deduct my portion—all after the check clears. I responded that no, I don’t pay your shipper for you, but you must prepay your shipper. He says that if I pay the shipper, I would advise the shipper when the item can be picked up and shipped to the buyer. Why couldn’t he advise his shipper. This has happened more than once. What the heck? I recognize that it appears that I can’t lose, but something bugs me. Anyone experienced this? Perhaps my suspicions were excessive.
Jerry