alt Hacker News> a monopoly on peer to peer sales of goods like that
I don't know ... around these parts (Santa Fe/ABQ) while Marketplace is very popular, Craigslist continues to be widely used for this, especially since an ever growing number of younger people are not on Facebook (either at all, or not regularly).
I would be just fine with a return to Craigslist but it's still mostly useless in my neck of the woods despite once being the main (digital) tool for p2p sales.