I built 3 stores in my settlement. I assigned one settler for each stall. They stand behind the stall during the day and when I check my workshop I do sometimes find bottlecaps which I assume are made from the stores.

However, when I walk up to a stall to buy/sell stuff nothing happens. I only get to trade with the settler like I normally would, as in I either give them items or take items.

When you build a store can you then not buy or trade your stuff in the same way as you would with a vendor in Diamond City?

Yes you can buy or trade your stuff like a regular store but depends on the store your built in the stuff you can buy.

How do you do it? I click on the store and nothing happens. I click on the settler assigned to the store and I only see what I would see trading with them without a store eg a weapon and ammo.

i have noticed that my stores will sometimes only trade. i think it has something to do with the time of day?? if the store will not barter, i just go sleep, or come back in a few hours (game time), and then they barter.

it is the time of day, it took me a while to figure this out but if you look at their opening hours on the side of the stand you'll notice that they remain at the stand for a bit after hours.

so if you talk to one while they're at the stand, but it's after hours you will only get to trade with them.

Come back (or sleep/wait) during their open hours and you will be able to barter with them.

It is working now thanks. I was visiting outside opening hours. Also, I think because I had named people assigned eg Cait caused problems, so I changed the shop owners to people called 'Settler'

How are the store owners total caps calculated? My general store and food store owners have about 250 caps each but my medic store owner has about 550?

It's a bug, obviously. The merchants at Sanctuary only offered me the trade option at some point. Stores in other settlements still work as intended.