...
So my morbid question is, what you intend to do with them as you get to old to use them? Sell on GB? Looking for ideas.
So my equally morbid presumption is that you mean before you're dead. Not too far from that myself. Not the dead part, I don't think, anyway ... the other one.
I think I'll just keep them and take them out to be admired and cleaned from time to time, I suppose. Unless and until I reach the point where I really need the money. I'm a good customer at a local mom 'n pop gunsmith/gun shop business and they've already sold a number of guns I didn't want or need on consignment for me, so I suppose I'll mostly go that route. Though I don't know what their expertise is wrt black powder muzzleloaders, so I might revisit that plan.
GB is a real pain in the you-know-what for the seller, unless you intend to get into selling multiple firearms, which these days is going to be pretty sketchy vis-a-vis the ATF if you sell and make a profit on your guns, and if you don't have an FFL, you might find yourself up on a felony charge.
I don't care what happens to them when I'm dead and gone. My estate can have the pleasures of dealing with that. Read: sarcasm.
So I guess base line my answer to your question is find yourself a reputable local gunsmith/gun shop, establish yourself as a repeat customer, and employ them to sell on consignment. If you just try to sell outright to a big-box sporting goods chain or to a pawn shop, you're only likely to realize 50% of current market value, whereas on a consignment you're more likely to realize 80% - 90%, depending on your agent's commission terms.