A little customer service goes a long way...many sellers wouldn't even think the way you are, so good for you for caring and wanting to make things right. :hug:
It's a gutsy thing to do, but you could ask them 'what can I do to make this right?' and see what they say. Of course, if they ask for world peace or $1000, that is extreme...but they might suggest something reasonable like what you had in mind OR they may say they don't know, what are you willing to do and you can suggest a remedy.
It was a honest 'mistake', so it is nothing to feel badly about. I mean, the person you sold it to is understandably upset, but as long as you care enough to make amends as best you can, it's all you can do.