Another option is SellYourMac (SYM). The process at SYM is nearly identical. Describe your iPhone, send it off and receive payment once it's been received and inspected. Both offer credit for accessories like a wall charger and earbuds.
If all that seems like too much work, you can take advantage of Radio Shack's trade-in program. Here's how it works. Complete the online form with details on your phone as with the other vendors. Or, bring it in to a Radio Shack store. They'll accept it and give you a gift card for the value of your phone, which you can use towards the purchase of an iPhone 4 (or a huge bag of alligator clips and AA batteries).
Prep your iPhone
No matter which retailer you choose, you must prep your iPhone for resale. It's a simple process, but it's important. Here's how to get your iPhone ready for its new owner.
Before shipping, you'll want to wipe your iPhone clean. The first, crucial step is to sync with iTunes and backup everything. Once that's done, click the Restore button (you'll be prompted once again to create a backup). Continue and restore your iPhone to its factory settings. All of your personal data will be removed.
You can also perform a restore with the iPhone itself. Select general settings and then tap restore. Note that this will erase everything, so do a backup first. It also takes a long time, so be sure that your battery is fully charged or, better yet, just plug in.
If you're super paranoid, you can do an iTunes restore followed by a general settings restore, but that's probably overkill.
What about the SIM card?
The iPhone 4 uses a MicroSIM while previous models do not. Once your new iPhone is activated, the old SIM will be de-activated. You can safely remove and discard it.
Your buyer expects his/her new iPhone to arrive safe and sound. If you don't have the original packaging (you saved it, right?), there are a few options. The US Post Office sells a great variety of inexpensive materials. If that's not enough, visit a UPS Store. They have a spray-in material that will conform to the shape of your item. It's pricey, so figure that into shipping.
If you're in the market for a used iPhone this week, you'll want to be wary of scammers. They're out there, but with a little diligence you can stay safe.
Be wary of transactions that require a money order or wire transfer. Secure methods of payment like credit cards are preferred. Also, question emails advertising a "free iPhone" or telling you that you've won one. Chances are you haven't.
Also, there are many people who wait in line for new iPhones so they can re-sell them for a premium. If you plan on paying a "scalper's premium" for an iPhone 4 (we suggest you don't), at least ensure that you will receive the phone.
There you have it -- how to successfully sell your old iPhone. Good luck, and have fun on Thursday.