This would be an interesting idea. There have been open source ecommerce solutions for a long time (Woocommerce is very popular), you could bolt AP to something like that and you'd have the next evolution of the Fediverse. Someone runs an instance for vintage clothing or fanzines or crafty things or art and individuals can set up their stores on it with the instance processing payments in return for a small cut to keep the lights on. Allow options like auctions or crowdfunding and things get even more interesting.