Is it (il)legal, according to copyright law, to sell the
personally-allowable backup of a given media as long as it is sold
with the original copy as a bundle?  For instance, I have considered
selling back issues of a magazine in such a way that an image-scan of
the magazine is made and burnt to CD so I could be able to read the
magazine on the computer -- a legitimate fair-use duplication while
still owning the original -- but then selling the magazine online
INCLUDING the disc I made as an incentive to buy the bundle.  Instead
of selling the copy, which would be illegal, the original is included
with the duplicate as a bundle -- would that nullify the illegality of
selling copies?

That would be breach of copyright.

Definitely not allowed.

I think the problem is in your assertion that the act of burning the
back issues to CD is "a legitimate fair-use duplication while still
owning the original."  US copyright law does not give you the right to
make a personal back-up archival copy of printed media like magazines.
 And even if you brought the act of burning the CD in under fair use,
you have no right to sell those copies since you vitiate fair use by
depriving the copyright owner of revenue (the buyer buys the copies
from you, not the copyright owner).  You may find it instructive to
compare the result if your question contemplated software instead of
magazines.  See 17 USC section 117
(http://caselaw.lp.findlaw.com/casecode/uscodes/17/chapters/1/sections/section_117.html).