The ToS is pretty simple regarding this, as stated multiple times by blues in the recent past:

If an item can be used ingame, then you can sell it for gold. If an item can not be used ingame, then you can sell it for cash.

So a code for a mount that you have to activate ingame is allowed to be traded for gold (ie talk to npc, then submit code and get reward)
A code for a mount that you have to activate in your bnet account page, can not be traded for gold, only for rl money.