Being realistic marketplaces are a little bit useless once you have the right contacts (speaking as a costumer), even if you multisig a purchase you can't really do much for protect yourself, the market has always your money and it can fraud you, or even worse, exit scam
I think $2 (that you can refund) are worth the service, is an easy straight way to keep in contacts with vendors
Sorry for my bad english
I think the info in Grams is based on either marketplaces providing an API for it to download listings through periodically, or Grams writing a custom scraper for that marketplace, and it's a little unrealistic to expect vendors to jump through hoops for yet another service. You'd be better off exhorting the markets to provide the API, since then all the vendors will be covered indefinitely.