Doesn't it make more sense for vendors to announce their offerings and users just contact them and pay the servers they trust most? Nostr is a good advertisement board, the rest can just be good old Rest API, just define that API. Even if you want to be a Nostr maximalist, then, users can send the offer to advertisers and pay in advance. Isn't it much easier to make a reputation system to 10 vendors vs 10000s users?