Not sure I understand the OP, but I as I understand this post, I would disagree that it would be 'logical' to *not* require OS disks. If you sell a system that you bought, that came with original grey OS disks, then by all means sell the system that way (I don't care what logic Apple may have otherwise). There is usually a Restore OS, and Install OS disk, along with the hardware test suite. This is not part of the usual separate retail version of the OS.
Therefore, include this with any system you are selling. Who cares what is actually *installed* on the system, the system could have the latest OS *installed* on it, but those disks that came with the system are the ones that the boot rom in that system will always boot up off of, incase you need to reformat/re-install or use Apple's Disk Utility to attempt a repair on a HD.
To me an used system (like the supposedly 'dealer refurbished' TiBook I just bought from Wegener Media, which has no disks included, neither retail OS or original set) without any OS DVD/CD disks has lesser value. Always nice to also have the user guide/manuals whether on CD/DVD or paper copy included. In short, the more complete the system being sold, as originally supplied by Apple, the higher the value, IMHO.