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.