Borella has your answer - You should not get banned for this, as selling diamonds in-game via the auction house is legitimate gameplay. If you do get banned for some reason, an appeal with support will correct the matter or provide you with the details of what you have done that is against the rules if you are banned for another reason.
Yes see that's what is iffy. I've had friends accused of breaking the rules by support when everything they've done is completely legitimate. The support for this game is cool and all but I can't help but get a really cruel dictator sort of vibe when nothing is stopping them from accusing you of literally anything whether you did or not, as well as nobody is really making sure that they're not enforcing some rule that's not in the ToS.
In other words, there's no checks and balances. I've seen people get banned/suspended and all their items wiped inspite of having paid over $1000-2000 in diamonds to Frogster. It's as if the fact that they paid that much in a short period of time is completely irrelevant, and that they can just accuse you of things and once they've made up their mind there's literally no arguing.... that is what I am afraid of.