I was wondering that as well.
Personally I think that would get annoying really quick. And I have yet to spend any caps on anything. I have enough crap stored that I can straight up trade enough of it for what I need. Usually what I get from merchants is ammo that I have a hard time finding. (.308 and shotgun shells) and rarer crafting materials. But the merchants so far haven't had anything that I just have to have. I can almost always build what I need or want from stuff I looted.