It's like trading in a car you own outright...one has to ask why? The convenience factor isnt even there really because selling on eBay is as easy as finishing an auction and driving down to the post office. Then you have Craigslist where you dont even have to deal with PayPal and online auction fees. Selling an iPhone is prob the easiest thing to sell on the planet because everyone wants one but nobody wants to be locked in a contract.
And some argue that the trade ins offer you a discount on the new phone you upgrade to...this is false because they rip you on the trade in of the old phone so basically, they make out better in all ways because that is the way their system works (just like a used car lot).
Sell it...be happy with more money...the end.