Android/iOS: You've probably already seen people you know selling stuff online through Facebook. Now the company has decided to get in on the action with the new Marketplace, which makes it easier to buy and sell with people near you. While this isn't Facebook's first foray into letting people sell stuff through its site, it seems to be one of the cleanest. Head to the new Marketplace tab (currently rolling out to the Android and iOS Facebook apps) and you'll see listings for things people are selling near you. You can search for specific items or browse categories like Household or Electronics.
If you want to sell something, you can make your listing relatively easily. Simply describe what you're selling, add a picture and a price, then give it a location. You don't need to enter your specific address (thankfully) but you'll need to meet up with the buyer at some point, so stay safe. The Marketplace gives buyers and sellers special messaging tools to communicate so you don't have to deal with letting potential buyers into your friends list or worry about privacy settings just to communicate.