How does Facebook Marketplace work?
Launched just a few months ago, Marketplace is only accessible via the little storefront icon on the bottom rail of the Facebook app (you can't get to it on desktop... yet).
Pull up the page and you'll see a selection of goods -- which you can search for directly or browse and filter via categories like Home, Vehicles, or Electronics -- based on items for sale within a one- to 100-mile radius of your (or any) location. Clicking an item brings you to its page, complete with more info, photos, pickup location, how long it's been on FB Marketplace, how many people have viewed its listing, and how responsive the seller is. Most importantly, though: It also links out to the seller's Facebook profile.
Of course, a seller's public Facebook profile doesn't necessarily provide the level of intel you'd need to decide whether you can fully trust a person, but it does at least put a face with a name before you even consider exchanging a message -- which is more than you can say for Craigslist. You can also see what, if any, other items the person is selling.
If you have more questions about an item, you can shoot the seller a message. But if you just want to buy it you simply click Make An Offer (you can submit any number you want). If the seller accepts, you'll coordinate pickup/delivery within the app. Once you meet up, you can easily facilitate payment via Facebook's own Venmo-esque encrypted money-sending service.