I suppose then, I'm not sure I see the point.
The purpose is to let people talk to each other about an item on eBay openly, right? You can do that now in a plethora of ways; post here, any gaming forum, reddit, etc..
There are a few holes. If it's an extension for instance, your only real method of monetization is something like patreon, or charging for the extension itself (good luck getting user buy-in on the latter).
If it's an app, you run into typical app nonsense, first of which is its *yet another app* and it has a very specific niche purpose, which relies very heavily on there being many users.
You'd have to manage how users find content, I'm having a hard time visualizing exactly how you're envisioning people finding content that needs to be replied to, which is also relevant to them.
Would it be limited to just video games? If so, how would you do so? Do you care at all about items that weren't listed properly (example: a video game posted under board games or something)?
I don't wanna shit on your thing or anything, I'm just thinking out loud really. Some stuff you may need to consider if you've not already. Naturally I wish you the best if it's something you want to do. I might spitball some more if you're interested in discussing further.