I've seen this in a few other posts, but haven't figured out what it's from just yet. It might be in the Settings>Security & Location>Apps With Usage Access menu. See what happens if you can select eBay in that list and disallow Usage Access.
My other thought is that this is just a courtesy notice to you that even though you're using a non-Google app (i.e., eBay), your voice search still has to go through Google's speech recognition service, which is then shared to the eBay app. If this is a new notification that no one ever saw before, then Google might have changed their policy recently to show this (in the spirit of being more transparent). Or maybe this API recently became available to app developers.