Was it on this account? (Asking as i can't see a sale for a single Spongebob game, only a bundle that contains the Squigglepants game).
I'm assuming, though, that the listing was similar to the other variation listings you currently have, in that buyers need to select a single title, which for most would be intuitive and self explanatory.... but not all.
Instead of calling the buyer stupid, it's more productive to take a second look at your listings and see if you can make any adjustments so that even someone who can't read would understand they are going to only get one game - you can consider this to be for your sake as much as it is any potential buyer's, as these situations (where buyer and seller are at complete disagreement) are best prevented (in as much as that is possible) rather than "cured". I can see the images adjust to show only the selected game when someone chooses a title, but I'd personally avoid using words in titles and descriptions that indicate the entire selection of titles (eg "All working"), and include clearer indications that the price is for 1 x Single Game - You Select, IYKWIM.
Personally, I'd accept the return purely as a goodwill gesture if nothing else - exactly how I would facilitate that would depend on whether I could take an objective look at my listing and see anything that would lead a reasonable person to presume the price was for all games - from what I know so far, that seems very unlikely (price, listing set up, etc all indicates single game to me), but your other listings have returns accepted so if that one did too, consider this a change of mind, which you accept anyway. I would be inclined to simply reply with a "You're more than welcome to return, please post to {address}, and I'll confirm when it has been received back as well as provide a refund of $x".
