Samsung's Paying New Galaxy Customers Up to $300 for Old Phones
The only smartphone that's worth the full $300 is Apple's 64GB iPhone 4S for AT&T. If you own the CDMA version, it will net you $250. The way it works is you request an instant quote from Samsung's upgrade website, purchase a new Samsung phone, and then mail in your old device within 30 days. Samsung will cut you a check for the agreed upon amount, provided what you sent in matched your description.
Android devices aren't worth quite as much, but can still bring in a pretty penny. HTC's Rezound, for example, is worth $200 if it's fully functional, without water damage, and doesn't have a cracked or otherwise busted screen. So is Motorola's Droid Razr Maxx. Nokia's Lumia 900 (Windows Phone) is also worth a pair of Benjamins, while HTC's One X will net your $175.