You can sell used CDs and used DVDs, and even, if you’re really old school, used records and tapes. So why not sell used MP3s that you don’t want any more. It may seem an obvious issue that an MP3 file can be copied again and again, but one company, ReDigi Inc., takes a different view of the issue, pointing out that the legal doctrine of “first use” allows purchasers of copyrighted material (such as songs on a CD) to resell them. Capital Records disagreed with ReDigi, and last October the two companies faced off in front of District Court Judge Richard Sullivan, in joint motions to dismiss the inevitable lawsuit that had been brought by Capitol Records. Last week Judge Sullivan ruled on the case, and chalked up one for the big guys.