Ebay counterfeit Windows 7?

Hi, i bought a new laptop in december on craigslist, the guy said it came with windows xp but he said he upgraded it to windows 7 so i bought from him and had no problems with it and a few months later i sold it on ebay to another guy who then said one month later that a window poped up and said that this windows is counterfeit. He e-mailed me and told me what had happened and said he would like my help before he contacts ebay. What should i do?