I'd be dubious about buying Spectrum codes from a guy you just met, there have been reports of people being scammed. If he's trying to sell them over XBL he's also breaking the XBL ToU found here: http://www.xbox.com/en-GB/Legal/LiveTOU
"Transferring consumable currency or virtual goods is strictly prohibited except where explicitly authorized within the Services. Outside of a game, you may not buy or sell any consumable currency or virtual goods for actual monetary instruments or otherwise exchange items for value. Any attempt to do so is in violation of this agreement."
You risk getting both yours and his account banned.
I would be hesitant about offering money to a stranger to purchase these codes. I received the spectrum camo through Microsoft's Expertzone program. After completing the accreditation I received a code for both the 360 and the One. I accidentally entered the 360 code on my One, and I received an error message that the code couldn't be redeemed on this console. My suggestion would be to pass on this one. All Gamestop and Best Buy employees have access to it, and I would suggest that you ask one of them if they have any extra.