If my business law memory serves me, the seller could not be the one to stop payment. That's the purpose of a cashier's check or a certified check....to get "good funds". But with a cashier's check, the BANK could put a stop on the payment, but it would usually be for some extraordinary situation. I'd call the issuing bank (or put the question to your bank and let them do it. Also let them explain this legality to you) and ask them why they took that action.
Good luck!
Good luck!