Let's pretend just for the sake of it that you wanted to seel one book.
You would exchange your right to have it, reading and all the benifits you would get fom having it and exchnage it for other kind of goods, usually money. Now, you have one game to sell and the manual, what would be the game's value?
(1) Playing the game itself, (2) having the manual in case some copy protection is needed or the game itself needs some kind of advice towrds playing it, and (3) original disks (4) original boxes, and althought (3) and (4) aren't needed for the actual act of playing it (as it will run in backups, adfs, whatever) they are there to present a physical value to the package.
From here we have that not having (3) the costumer might value a litle less, or perhaps not, it's up to him. If one would sell the original how would one know that the previous owner didn't made a backup?
My advice... sell one book for the price and give away free your backup disk! ;-)