Unfortunately, you can't "put in a claim against the buyer" because it is no longer sealed. Buyers are allowed to unseal their purchases. If they weren't, how could they return a genuinely faulty item?

You just need to work out if its value will have dropped more than £70, so would it be worth just going along with the buyer, even though you know he's "at it".

Personally, I would insist on the return, even if I did lose out financially. Regarding the codes, I would explore whether you could cancel them, but that may well not be possible. After all, although you bought them, you didn't activate them. To Nintendo, it would just be the same as you cancelling a gift, and they won't go along with that.

Sadly, whatever happens, it looks like you are going to lose out.