Some games in my collection are missing manuals. From most of those games I knew that before I bought them. I figured that I would get my hands on a manual at some point. And it provided me with a somewhat cheap way to obtain those games.
Two of those games are Wasteland and Goblins 3. Both games usually sell for more than one hundred euros or dollars and I both got them for way less, excluding the manuals. Wasteland I got almost three years ago and Goblins 3 almost two years ago. Now within one week manuals for both games turned up. Now I can finally turn those alerts off.
Ultimately I think it has cost me about the same as complete copies would have cost me. I comfort myself by thinking that I saved those poor orphaned boxes from being thrown out.