Without doing the sales pitch, it's using comments as the medium for indicating purchase intent instead of clicking on a link

"Oh, you like this item you see on your facebook feed? Comment sold and your size to order it"

Ah. Makes a lot of sense. Gets the person to a lower friction commitment point where they feel like they’re going back on their word if they abandon at that point. And you can harass them with PMs to follow up lol.