You have rights, but when you do not adhere to policy nor follow the rules, you cannot complain about Amazon not treating sellers fairly.
A seller, regardless of being on ‘vacation’ or not, are required to respond to messages within 24 hours. When a claim is filed, a seller has 3 days to provide a response to it. Failure to respond to a claim, even if Amazon agreed with you, they are forced to follow policy and find for the buyer.
If I purchased from a seller and was told that my return would be processed when they come back from Africa, I would not be happy either. I am not going to comment on the fact of selling ‘Access Codes’ because your post does not provide enough information to know whether you are or not allowed to sell it since Amazon does have strict requirements about selling such items.
Also, the statement “listing states that I can’t accept returns on access codes” is a violation since you cannot set your own return conditions. When you agreed to sell on Amazon, you agreed to meet or exceed Amazon’s return policies. Simply ignoring that fact and violating policy by stating a return policy on listings that violate Amazon’s policy is not acceptable.
I agree that Amazon can act unfair at times, but I have little sympathy when a seller that does not follow the rules complains about it.
If you cannot be available once a day to check your account and respond to messages on a consistent basis. If you are not going to follow listing policy and adhere to Amazon’s required return policies, then Amazon is not a good platform for you to sell on.