"They're good for the environment and good for the budget," Katherine Hutt, a spokeswoman for the Better Business Bureau, said of companies that buy and refurbish iPhones. "But know who you're dealing with." 

Given the amount of legwork that goes into checking up on resellers, you may find it easier to sell your unwanted iPhone yourself, either through Craigslist or eBay. It could be more financially rewarding, too, as you can set your own price.

No matter how you sell your old iPhone--whether via a reseller or all by yourself--you'll want to make sure that the phone is stripped of any personal data before it leaves your hands. Apple has some pretty . Just go to the Settings -> General -> Reset, and touch the Erase All Content and Settings button. You'll also want to guarantee you've backed up all the data on the device first, so that you can restore it to your new phone. iTunes should do that when you sync it before buying your new phone, but to be extra sure, select the iPhone in iTunes' Source list, right-click on it, and choose Back Up.

Giving your old iPhone to a friend or relative may have somewhat less allure now that Apple and the phone companies will be (provided, of course, that you sign a new two-year service contract). But an iPhone 4 will still make a nice hand-me-down, especially for people who don't want to pay $99 to buy a new iPhone 4. Before handing off the phone, though, make sure that your lucky recipient really wants to take on a two-year commitment with a wireless carrier. And perform the phone back-up and erasing steps outlined above.