If you really aren't interested in the phone, leave it closed box. As @EskeRahnsaid, FxTec will honor the warranty for the new owner.
If you don't know if you want the device or not, but are worried that open box and tested will lower the value, I wouldn't worry. Take it out, try it out, see if it might be something you want after all. I don't think there is any reason to charge less for open box and tested (again, the warranty will still be good for the new owner), and, who knows, maybe you'll change your mind. 😉