I sold a game recently and gave the buyer the manual with it (Williams). He commented that it was rare that someone actually gave the manual with the game being sold. Come to think of it, I've witnessed the same. I've actually bought games from people in the past who had it sitting on the counter, asked for it, and they said not for sale with game - that is for another one they have coming in or borrowed from a friend.
My perspective, it should be part of the game sale if you have it. Thousand plus games being sold and you really hold back on the manual to post for sale for $15-$30 later? Interested in thoughts.