Uh, I suppose there is a high density of coin shops where I live then. I can think of three within a 15 minutes drive from my house in Italy and a few in London too.
Well it's up to him, of course. If he is willing to make the deal, he will wait. If he isn't even after I offer to pay twice as much, probably either he is not so desperate or he is a scammer.
If he wants to sell the coin for $50, then he doesn't know its real value. If he doesn't know its real value, he won't be concerned with me going inside a shop and have it checked. If what you are saying is that I should not trust the coin shop owner, I thought it's pretty obvious that I'd sell the coin straight away, go out and pay the guy his $100. I can't see any serious risk in this approach.
As for your comment, I find it pretty funny for a very personal reason: pretty much everyone close to me points out how hard it is to gain my trust
Which is both a compliment and a criticism, no?