Aion Wing Code

Would it be against the rules to buy/sell an Aion Wing code for ingame cash

yes

It is against the rules to sell game codes for in-game cash.
That isn't to say you can't give it away, which a good number of people did.

The reason it is not allowed is because it is open to scamming. Who goes first? The person with the code or the person with the gold? Whoever goes first can't be sure that the other party will complete the deal.

yeah but beware if they have the code they could email anet and give the Aion code as the game code and potential take ure account, be warned as this happened to me. but as Red Dog said yes they can be given away for free but not sold.