My guess is that the seller won't test because it might be broken. If it is broken, and he says so, the bids will be lower. Does that make him a crook?
On the other hand, since all the bidders know about the unit is that the VU meters don't light up and it 'might' be broken, then they intelligently should bid as if it *is* broken. But they won't, will they? No, they'll bid as if the unit works, because they hope it does. Does that make them stupid?
I understand the frustration - if the thing is busted, an intelligent potential buyer wants to know so they don't bid too high. If it works, they want to know that, so they can raise their bid accordingly. And of course, the intelligent bidder will be outbid on this item, because the idiot bidders will outbid them, since in their minds, the unit works.
So the idiots win.
And this is actually how the world works. The idiots always win. We just usually don't see such a graphic example of it.
Walk away. Bugging the seller to test something he has already explicitly stated he won't test is not going to do any good.