Advanced shop stand. Can set a limit to the number of purchases per customer (character) when giving away cheap/free items etc. although they could still come back on alts. Or you could limit 5 per customer per day when having sales to promote a shop and prevent someone just buying it all. Could also have other advanced shopkeeping options, such as “Only sell THIS hammer or these diamonds to SirPwnz.” which would allow offline trading.
People could also do front end requests for places like The Stone Shop. They’d be able to post orders for shop keepers to fill, kinda like a reverse request basket. Walk up to the kiosk and place an order for 5,000 pale yellow metamorphic stone or whatever… and then the shop keeper either accepts or declines the request. Some other stuff happens after that but it’s basically a deal making basket.