Muzzleloading guns are not considered as "firearms" under Federal statutes. You may sell them as you wish, and even ship them through the post office system without restriction. A very few States have modifying regulations regarding the firearm/muzzleloader thing. I don't know if NH is one of those..
12 GA New Englanders are in fairly high demand. If it's in good shape I suspect someone on this forum will be sending you a private message asking for pictures and offering a fair price. I would guess something like $300, with shipping cost negotiable as to whether the buyer or seller pays.
My best guess on the Investarm .50 is around $250 (plus shipping) to $275 (shipping negotiable) so long as it's in reasonable shape with a clean bore. A rusty/pitted bore knocks it down to something like $125.
I'm not up to date on the Knight as I don't follow in-lines much.
Anyway, that's my take. Others may well disagree with my value estimates because muzzleloaders in different areas of the country have widely varying popularity and demand.