Apple is expected to announce its new iPhone at a September 12 event. After every major Apple announcement you usually see a flood of older model devices for sale on eBay and Craigslist. iPhone 4 and iPhone 4S owners can rejoice because Apple is saving you a trip to the post office.
Old iPhones can now be traded in directly to Apple and they are offering a good amount of money with their Reuse and Recycle Program. Currently an iPhone 4S 64GB is valued at $345, the iPhone 4 16GB will get you $160 even if it is slightly scuffed. Apple is not giving their customers back cash or check for their trade-ins, instead you receive the funds on an Apple gift certificate. Details about Apple's trade-in program can be viewed here on Apple's website