If you really want to mess with their heads, say - "go ahead and send the check" - then when you get it , tell them your bank advised you to wait until it clears before sending them any money. Or tell them the bank said it was fake.
Or tell them you cashed the check and decided to keep all the money for yourself because you had unexpected bills.
I have gotten these kinds of responses on items as low as a $20 ice cream maker. Now I just usually disable email replies on a CL ad - they can call me if they are interested.