While shops and NPCs are still a bit hard to decide whether they will be treated as action, separate menu or location, one thing is for sure, that shops need specific stock to be set realistically. This will greatly supply the idea of local prices. So:
shop should have stock written in their respective JSON-like ID, being list of IDs (can repeat) of items on sale
buying the item make it disappear from list, hence not allowing player to buy that exact item again, if there was only one of it
shop should have "last date of visiting" field that indicates if restock is possible
if shop has less items than
restock picks randomly/missing items from "possible stock" list
shop configuration files, if we make separate ones, should have "IDs of items" instead of predeterminet stocks: stock will be dynamically set once first visit, and then resupplied, but exact set will be unknown, as those will get distributed randomly (with possibly some algorithm to even things out, so it won't be 10x of X, with 0x of Y)
prices and NPCs sympathy matter, but how those will interact is still up to talk (possibly binding shop with entity ID and outside location?)
While shops and NPCs are still a bit hard to decide whether they will be treated as
action, separatemenuorlocation, one thing is for sure, that shops need specific stock to be set realistically. This will greatly supply the idea of local prices. So: