What Would You Do?

Realistically, offer him $30 (or $20, or $10, or $5, depending on when in the day it is and what I intended to do that day) because I'm highly unlikely to have so much money nearby. Then say that if he can wait a few hours while I go fetch it, I would give him much more money (probably between $200 and $500).

If I actually have $100 on hand (say, if I already went out to buy some coins), probably something similar to option 4. Specifically, if I ever sell any of his coins, I'll try to give him at least half of the money (and yes, I'll probably say it initially).

Just in case: if he ultimately doesn't want to sell it anymore - well, I would just walk away sadly but won't be particularly worried. More than once I had found good deals on coins, and almost every time I specifically asked if the guy really still wants to sell that coin for the price they gave (and they almost always did - I'm not sure if I've ever received a negative reply).
...I'll try to make sure that at least I get my $100 back, however.

I had a dream or a day dream similar to this poll;

What if, you were given the opportunity to buy someone's inheritance but there were no really big money coins. Maybe a bunch of Indian cents, wheats, maybe a few silver dollars etc...

But there are rolls of cents, and you offer a fair price for the lot, but without looking in the rolls (you assume all pennies) Your in a hurry, the sellers in a hurry, no time to search through all the rolls.

You go on your way, and later you open the rolls, and find pennies, but you also find in it a gold piece.

I contemplated this, what would I do? Go back days later explaining what happened, perhaps offer more? Maybe just chalk it up to "Bank error in my favor" and take the win?

Its an interesting scenario, and a dream for most of us