I also wonder if the purchaser might be just as interested in the frame/box as he is in the photograph. Yes, I'd get it appraised.

The seller wasn't asking for some strange pay arrangement, was he? Like he'd send you a check for more than the amount and you could send him back the extra in cash, etc. -- i.e., a scammer, offering you an amount you'd jump to get so you'd be willing to overlook odd pay arrangements?