I think anyone who buys a digital music collection would want to be sure they are buying legal music. If that's the case then I would expect the digital storage media (USB or SATA HDD or SSD or whatever) would also come with
a. Physical CD for each album ripped from disc
b. Email receipts for purchased digital downloads
c. Signed & dated document stating that any other digital copies of downloaded files have been deleted
That would give the buyer reasonable assurance and proof that he has rights to the files and was not purchasing illegal copies.
