you must include the Windows CD/DVD
Wrong. PCs do not come with a disc. That is sooooo 1990's....
They come with recovery drives now, Terry.
But he shouldn't be selling them, plural, all with the same key, it can only be legally used on one. Unless it's a VLK (volume license key) in which case his selling it would be illegal.
If the guy on CL is just an individual selling off a few used PCs (or newly built, for that matter), I know of no rule/law that says the PC needs to have any such sticker on it. Especially if, like mine, they're PCs he built himself.