Swappa focuses almost exclusively on smartphones and tablets of all types, and has a huge userbase of people both selling and buying devices in good, workable condition. Selling a used phone on Swappa is super-easy, and only requires that you verify your account, take some pictures of the device, post your listing, and get it approved by the Swappa staff. Swappa only deals in clean devices as long as you buy from a trustworthy, verified seller, so you don't have to worry that the phone you buy is stolen or won't be able to be activated. Buyers get the satisfaction of knowing that the phone or tablet they get is in good working order at a great price, and the sale is never closed until the buyer has the item in hand, inspected it, made sure it works and activates, and they're good to go. Swappa also handles things like payment and dispute resolution, which makes it a pretty good platform for buyers and sellers.
Of course, Swappa isn't perfect either. Their fees are lower than sites like eBay, which is great, and they have a large user base of people who are looking for great deals on phones—along with tons of protections for both buyers and sellers—but there are tons of people on the site with sketchy lowball offers and people selling dozens of the same phone who look like they may have lifted them from a warehouse or something.