You ought not to mark it 'payment sent' as that will not nullify the transaction. To cancel, the seller has to send you a request for cancellation. I do not believe this seller will spring an Unpaid case on you, however, as he has no listings up right now meaning he is either taking vacation or closing his account(s) entirely. A seller has only 32 days in which to file one of those.
Ordinarily you could always find the transaction in your paypal and send a seller the money that way, but in this instance I wouldn't if it were me. With no useful communication from the seller, it is not likely you would receive your item.
You might try one more time to simply ask the seller for a cancellation. Be as specific, clear, and concise as possible, - and always polite. Same as when you call pp customer service.