Actually when you read it closely, there is the implication that if you sell your own disk w/o any copying of the songs, it may be legal under the first sale doctrine.
ReDigi's problem was that the resale was from the cloud; the ReDigi system transferred the song to the cloud from where the sale was made. Notwithstanding the fact that the original "at user" file was destroyed, the court ruled that the transfer to the cloud was an unlawful copy.
Since the cloud version was an unlawful copy, the first use defense cannot be used to protect what is deemed to be an unlawful copy. The case isn't entirely on point as to the original question.
The case does not make any mention if a sale of a lawful copy is or is not covered by the first sale doctrine.
Sorry about the nitpick....