You have a very nice rifle there!
It's hard to tell from these pictures, but it looks like it has a full length barrel, and could be in original flintlock configuration, which would be a plus. Do you have any idea what the hole is just below the pan?
Sinc your Grandpa proved it safe to shoot, that would be a super fun gun. If you haven't already, run the ramrod down the barrel to be sure it isn't still loaded. Just to be safe, if you're not familiar with these old guns, have a gunsmith check out the bore/breech plug condition to be sure it's safe to shoot.
The one in the auction link sold for $650, but yours in this condition I'd say is worth more along $2000/2500+ range. Just take care of it, don't refinish it, and don't try to cover all the old nicks and bruises. Just use it, enjoy it, and count yourself lucky!
John