I get your point and in most cases misseable are kind of unimportant to the progress, but there some that are kind of important to beat bosses.I mean you can beat every without needing one special item, but some missable made some very hard bossfights a lot easier. I mean like the challenge a lot and like the fights to be even harder, as darksouls player, but for players don't like to suffer, it's kind of bad.Aside from that i kind of like to collect all the equipment in a game.
So the idea of a new trader that sells missable items is very good, as long as the prices go competly crazy, like 30k for one item, i think somewhere between 1k and 5k seem reasonable, but i the end it's down to you decide the prices.