DS Game sold - buyer saying it doesn't work

I've just sold 9 ninentdo DS games, all were checked and worked perfectly before selling. Most had hardly been used. I've just had this from one of the buyers'Hi There, I received The Ben 10 and Simpsons DS Game. The ben 10 game is working fine but the simpsons game will not work? My husband says that sometimes that happens it will play on one and not the other, However Its no use to my son if he cant play it. Can I therfor send it back for a refund?'I know that the game worked fine, am I in my right to say that I don't have to give a refund? If it's their DS, surely that's not my problem. I've got nearly 500 feedback with a 100% record and do not lie or falsely sell items that aren't working.Any help would be appreciated.Thanks

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