Wikipedia
ReDigi
ReDigi is an online marketplace for pre-owned digital music and the only cloud storage service that verifies whether each music file uploaded for storage was legally acquired from an eligible source. Currently ReDigi's Cloud and Marketplace only accept iTunes purchased music.
ReDigi does not buy pre-owned digital music from its users. Rather, ReDigi's system is set up in a way that allows users to buy and sell pre-owned digital music directly from one user to another.