Can I sell new unopened AT&T iPhone 5 bought with upgrade?

A few months ago I got an iPhone 5 with an upgrade and now again have the opportunity to purchase another iPhone 5 at the upgraded price. If I order this second iPhone 5 and sell it unopened factory sealed would there be an issue/problem for buyer or myself?

So much as you do not end up with a delinquent account, there will be no issue with selling the unopened phone.