ebay is not a 1st party seller. Its all third party. Some first parties sell like target but target is already included in your list.
Same with macrumors and reddit. Those are sold by individual people, thus making it third party.
I suspect that list was not so much intended to be seller companies who got their bands direct from Apple, as places where one could buy genuine Apple (i.e. 1st party) bands, as opposed to other companies manufacturing bands (i.e. 3rd party), whether they're AW-compatible bands, knockoffs, or actual counterfeits. You can, indeed, buy genuine Apple bands from eBay, Reddit, and the like, but your overall odds of getting an actual Apple band are less than if you walked into an Apple Store. OTOH, your chances of getting a legit band in a discontinued color are higher on Reddit than at Apple Store (because they simply no longer have it).