A frequent distinction between the two situations is that usually, you buy the actual physical thing (DVD, VHS, CD, book, etc.), but for a purely digital asset, you may be only buying a restricted licence to the asset (on iTunes, for example).
In the case where you did buy the digital asset (not simply a restricted licence to it), another difference is that the transfer of a physical item does not implicate the reproduction right1 that is given exclusively to copyright owners. However, transfer of a digital asset often does implicate that reproduction right. See Capitol Records, LLC v. ReDigi Inc.:
It is also undisputed that
Capitol did not approve the reproduction or
distribution of its copyrighted recordings on
ReDigi’s website. Thus, if digital music files
are “reproduce[d]” and “distribute[d]” on
ReDigi’s website within the meaning of the
Copyright Act, Capitol’s copyrights have
It is a different situation if you transfer the actual medium onto which you downloaded the digital content (ibid.):
Section 109(a) still
protects a lawful owner’s sale of her
“particular” phonorecord, be it a computer
hard disk, iPod, or other memory device
onto which the file was originally
1. 17 USC §106 (1) "the owner of copyright under this title has the exclusive rights to [...] reproduce the copyrighted work..."
This answer assumes US jurisdiction, but Canada's Copyright Act is fairly similar to 17 USC and similar holdings have been made in Canada.