I am neither; "Copyright Specialists and Manufacturers who claim this protection", but have asked this question of both of them.
As I understand it {IANL} the answer is the "-copy-" part of the 'right to copy' (copyright), in this electronic age.
Almost without question, you would not get in trouble if you had properly purchased a hard copy of a manual, and then sold that hard copy via e-bay or your local flea market, since you were not _copying_ the manual. same as the Harry Potter book at your flea market. It is when we electronically 'copy" a *.pdf or other electronic format file, without the specific permission of whomever owns the original information, that we break "copyright".