OK, I think I misunderstood and maybe I still don’t understand. I thought you meant you wanted a buy button for a product, so users could purchase an item. Now I think you mean that you want user who are adding items to be able to purchase an upgrade to make their listing a featured listing. I’m not sure I fully understand how you want this integrated with favorites, but I suppose you can set visibility on the Favorite component when a listing is set as featured.
As for purchasing an upgrade to a featured listing, I think it would be best to let the user create the listing, then you create a buy button on the listing detail screen. Set visibility on the buy button to only show if the signed in user is the user that created the listing. Once they purchase an upgrade, you can use a VLOOKUP or some other formula in the sheet to set the Featured column to true if a matching record is found in the App:Sales sheet. This will allow the favorites components to show for that listing.