Looks legit to me to be honest. You brought it, have used it, its working... why are you worried? Might get one of these myself. Is this just an upgrade version or can you reformat and install windows completely and not just upgrade ontop of Windows 7?
I paid £41 on google shopping, clean install too. I went to a Microsoft page link, entered my key and was given a download link to make a bootable usb /dvd .iso.
I wouldn't be buying keys from eBay, don't care how much good feedback someone has got, if someone else has/had access to that key, it's a timebomb until its blacklisted/used up all its activations @OP - You're not the eBay seller by any chance are you? Advertising your sale? lol