I have blocked this buyer after this realization but this type behaviors leads to more bad than good. The original transaction from me is complete but it is doubtful the code was confirmed or redeemed as he is re-selling it.
You can refer to this topic regarding reselling.
You can state in the description that it should be for own use only and that it should be used immediately.
I fail to see any problem in re-selling. Despite the fact that your codes are confirmed to be working, which is great. There is nothing precluding the re-seller from doing the same.
There are many reasons for reselling and it is not an absolute certainty that the person you sold your code to is re-selling your specific code. There have been several instances when I was waiting for a code to be delivered that was delayed or originally not working, and because I had a sense of urgency I purchased another code then simply re-sold the original code when it finally arrived or when the original seller made good by providing another code. It is also possible to earn digital items after purchasing the same. There is nothing nefarious about that.
Commerce works as commerce works. Unless they are the manufacturer, a seller is always buying before they sell.