They don't, this is just random squatters occupying land that isn't theirs. This is basically on par with finding a vacant lot that the owner doesn't check up on and "selling" it while openly admitting that they don't have title to the land and have no rights to the property. What's surprising is that people are foolish enough to pay them anything. They aren't actually buying anything, they could just not give them a dime and go extract the lumber, slash and burn the lot, etc.

How is this different than someone selling the Brooklyn bridge on Craigslist?

It's not. And you could offer the Brooklyn Bridge on Facebook. Transactions don't take place on Facebook. If I browse Facebook for a basketball, we meet up and they want to hand me a soccer ball, I don't give them cash. If you're conducting a trade for land/property, you need to go through all the processes normally involved in property transfer -- survey, title insurance, etc...

Facebook marketplace just links buyers and sellers. Settlement is something else.

Why is everyone so intent on Facebook/Twitter/whatever being the arbiter of truth? These problems aren't new to any of the new tech.