I did the same thing with my wife's iphone4 when she got her new iphone5. I am using it for myself on tmobile. However, I quickly discovered an issue with one particular app, textnow+ where even after doing the reset and setting up the phone for my appleid, using textnow causes a conflict between our two iPhones. The bad part is that we can read each others text messages because textnow must have a log on its servers with the udid of both phones. Whenever I try to log into textnow with my account, it logs her out on her phone and logs in as me, and vice versa. I've contacted Enflick weeks ago with no response. I've also posted to these forums with no response.
It seems like a major security flaw to me. I'm not sure it's only limited to textnow, but it's all I've experienced.
So, maybe selling an iphone isn't totally without risk, in my experience.