Okay I'm gonna say it, can't he just use that luck potion himself, and while it's in effect, use that to sell even more luck potions? I feel like every gamer or RPG player would've thought of that.
I guess this is one problem with including "luck" in stories, it can make characters (and the author) seem stupid if they didn't think of that, or if they did they REALLY need a good excuse why it wouldn't work.