Yes. Assuming there are no contracts to the contrary (hopefully, you’re not trying to sell a rental), you can sell your phone. But there’s one odd caveat.
Why are we even asking this? One reason to think about it in the context of copyright is that the phone will contain copyrighted software on it. Even if you uninstall all of your apps, wipe the storage, and cancel your services (all of which and more you definitely should do before handing it over), the phone will likely still be carrying copyrighted software in the form of the operating system and firmware that makes it run. But if you hand those programs over with the phone that holds them, you should be in the clear—just as if you had handed over a CD, DVD, or Blu-Ray to a buyer.
But that odd caveat has to do with the question of ownership: do you actually own the copy of the software that sits on the phone? If some manufacturers are to be believed, you don’t—if the license agreement you probably clicked on when you activated the phone said you weren’t the owner, then you weren’t. And because only the people who own copies of computer programs can distribute them, you might technically get in trouble for it, by being sued for copyright infringement.
This isn’t something that is likely to come up in the average person-to-person sale, but it’s enough of a legal hook for someone who might want to prevent large-scale resale of phones—maybe a wireless carrier or a manufacturer. The copyright argument is just plausible enough to use to go after an online marketplace, or a big reseller or recycler, who the companies might not appreciate undercutting their sales of new phones and service.