In a blow to the world of music downloading, a federal judge has ruled that the reselling of digital products constitutes copyright infringement. The case concerned a startup named ReDigi that allowed listeners to buy and sell used music originally purchased through iTunes. ReDigi said it deleted the original music file, making its system similar to the way a used bookstore would resell books. But U.S. District Judge Richard Sullivan sided with Capitol Records in its lawsuit against ReDigi, ruling that the so-called first-sale doctrine — which allows someone to resell a copyrighted book, for example — does not apply to digital music files. ReDigi says it will appeal the ruling.