You are allowed to sell them in the upgrades section, just inform people that they are for 3 months and are pre-paid.
Then people will add you on MSN and you can either give them the code over MSN first (only recommended if they're highly trusted) or you can apply them via teamviewer or ask for their password and username and do it yourself.
Last 2 ways are ways to avoid that people can say that the code was invalid.