If you're selling to a friend then maybe they'd want a code. I know if I were on CL then I wouldn't risk that the code was already used and plus you'd have to wait a week or likely more for shipping. If you have good eBay FB then maybe it'd be worth the risk but I'm not sure. I'd say most buyers would rather design it themselves but whether they think it's worth the risk or not I'm not sure.
I'd rather be able to design the x, but selling an actual phone would probably be easier. People may think its a scam.... Unless u have some very good feed back or someone that knows u is buying it.