ReDigi, a company that re-sells music files in the same way old CDs or books are sold, is being sued in America by big music publisher EMI.
EMI don't think people should be able to re-sell mp3s they no longer want because there's no way of proving that the seller hasn't kept a copy of the file.
But ReDigi say their software runs "continuously" in the background on a user's computer to check for copies of songs.
A court in New York, will hear both arguments this week.
The case will be watched closely by music companies around the world because the decision made in New York could set the standard for the whole industry.