I think I've looked at it maybe twice since it launched, to admire all the weird scams. Maybe it's gotten better since? It used to be sub-ebay levels of complete nonsense.
Maybe the gatekeeper thing is a reflection of less people in the EU using FB at all, rather than specifically Marketplace.
It has taken the place of Craigslist for younger generations.
Facebook literally is the internet for millions of people.
Facebook marketplace is far larger than craigslist and ebay combined, even if you take both of those at their respective peaks.
The open web might seem huge, but it's actually dwarfed in size by Facebook.
It’s the classic disconnect between engineering and product management: When engineers don’t want a product and therefore conclude that nobody wants the product.
When I’ve brought up Facebook active user stats here in the past I got flooded with responses suggested Facebook was lying or manipulating their user counts to pump up the stock.
My family uses its messenger for organizing things because everyone has it, even if some of us rarely use it except for that. If I wanted to draw attention to something locally, whether it was promoting a service or running for office, I'd be a fool not to use Facebook.
Like, fifteen years ago, if you happened upon the Facebook page of a random person, you'd usually see a handful of vacation pictures, a meme or three, some updates from their latest Clash of Cookie Farm Kitchen Dash session, and whatnot.
These days, all that stuff - if it's even still being posted - is likely siloed away to Friends-only posts. That random person might still be there, might still be logging in every day, but you don't see the Messenger group chats and the Marketplace offers/haggles.
Likewise for small businesses - a lot of the "look at this thing we're selling now, come check it out" posts now go to Instagram. They might still be auto-logging in, still responding to PMs on Facebook, still clicking a few news posts here and there, but that's just not visible on the outside, and creates the perception of Facebook the Ghost Town.