IANAL but I don't see that a violation. As long as he isn't selling or renting it out for a price then he is fine. I have a feeling that provision is there to make sure people don't try to sell credits, screw over the buyer, then the buyer tries to complain to Amazon.

Steam has a similar stipulation - and a lot of people are pissed about it. Basically you can't buy or sell steam accounts - and if you are caught Valve will suspend the account. I personally think it's completely stupid and if actually challenged would not hold up in court (I think the first sale doctrine fits perfectly because if you sell your steam account then the games are no longer accessible to you). But again - I get why, they don't want to put up with scammers or stupid people who try to sell an account and disappear before they send the credentials.