I really hope they don't poison that market as well.
Well, to be fair, there are digital product marketplaces that are still essentially monopolies (DriveThru, Kindle). I doubt Facebook is going to take over those markets any time soon.
What products are there currently for doing this beside Gumroad?
DriveThruRPG and its sister sites are the ones that stand out to me. But I operate in the tabletop RPG industry, which is fairly niche.