GOG's site has no ties to ActivityPub, but one element from there may facilitate building on that idea.

There, they have a review system, but it's not locked to only buyers. As long as the page for a game, DLC, etc. is up, you can leave a review. And as a way to mitigate voting manipulation, you as an user can filter by owner reviews specifically (and iirc, filters are saved in the browser's session so you don't need to change all the time).

Now, into ActivityPub, two store projects exist that I know, Bandwagon and Flohtmarkt, respectively trying to mimic Bandcamp and Facebook Marketplace's experiences. Unsure how they federate their contents or how they work internally as I haven't tested them yet, but if a product listing gets propagated as a post to other social medias, people could comment on them. But if internally they can check who owns the product of the given listing, they could do so to filter out all non-owner comments.

And furthermore, borrowing ideas from yet another store, the smut manga store Fakku allows people to comment on product listings, and optionally leave a rating along with the comment. Again, if either Bandwagon and Flohtmarkt have a way to check ownership, or if another ActivityPub-compatible store engine is released which then could have such check, maybe the store/engine could make the rating part locked out from non-buyers.

And lastly, engine-specific functions don't seem far-fetched. From what I read, PieFed has things like community migration and Reddit-like flairs, Misskey has pretty powerful client-sided CSS capabilities, Mbin mixes microblogging and threads pretty organically, Minds, Wafrn and to an extent Friendica connect to multiple protocols, and Peertube feels like its own thing. So such proposed ideas, if not already applied, are within the realm of possibility.