You should end the original listing, then relist with changes.
Make the new listing Fixed Price / 30 Days, with Paypal only and Immediate Payment required. Change the asking price to that agreed upon.
Change the title to the customer’s name. Give him a deadline to pay. (Perhaps 48 hours at most.)
If he pays. Proceed as usual.
If he does not pay within the deadline, Block his ID, and change the listing to the price you actually wanted and the title to the original.
This may sound a little harsh, but often these offers are from sketchey customers, who are either trying to get rid of a competitor or are just messing about. That the offer is LOWER than your asking price is, if not a red flag, pinkish.
