Thanks everyone for the input. I've been in touch with the seller, he doesn't know the details other than that picking a third-party CC processor is a choice for payment. He informed me he had sold things before and the processor receives the payment, takes a percentage, and then he eventually receives the payment. Audiogon was helpful in providing some feedback on the seller (he has sold things before and received very positive ratings), but clear information about payment was still not solid (suggestion to use Paypal if seller was willing).
I'm communicating with the seller and making determinations through some careful questioning and arrangements. I believe he at least is legitimate / trustworthy. Still not sure I'm going to buy it yet. Working on some means to provide protection for myself in the event of an unpleasant issue.