Don't feel bad at all for returning an unopened phone which they will turn around and sell to the next guy at full price.
Even though it wasn't opened, the device cant, and won't, be sold as new anymore. T-Mobile opens and visually inspects each returned device whether it's still sealed or not. Also, once the IMEI is sold its no longer available in their system to sell it new again. All devices go to their warehouse in Texas to be recertified and sold as a refurbished device or returned to the manufacturer.
I won't comment on what your doing as it's none of my business.