Can I sell my iPhone 6 plus, upgraded purchase?

I upgraded to an iPhone 6 plus through ATT. The phone is coming in a week, can I sell it on eBay for a profit? Is there anything tied into that phone prohibiting me from doing so? An activation code or perhaps something else?

Added (1). Let me be more specific, I upgraded through ATT, how does a person who buys my phone activate it?

It will be locked to AT&T, which severely limits the ability to sell the phone. And unless you bought it outright, you're still on the hook for a financed phone even if you don't have it any more.

The iPhone 6 is officially released and widely available. The only way to get more than retail for it is to have it unlocked and sell it to someone in a country where it's not yet available. Time is short.

If it was paid in full, nothing to stop you from selling it. If you are paying monthly on it, the IMEI or ESN, will show up as Financed, when the buyer goes to activate it on a cell phone carrier, making the phone useable. So if a smart buyer (wanting to spend extra money), to buy your phone, asks for the IMEI/ESN and you refuse to give it, this will alert them that something is up, and they will walk from the deal, again if it in NOT paid for.