Thanks for the advice, this is very helpful. I'm waiting to hear today from my friend who is local about scheduling some time with the seller and seeing the equipment in person and testing it. I'll mention the Q Audio option as a neutral ground place to bring the components in and plug them in (and I'm definitely willing to pay a fee for the shop's time to facilitate this).
Will see how the seller responds and if more red flags come up or objections that seem to hint at something being not quite right.

