Why are used laptops sold with no OS?

I'm shopping for a cheap rarely-to-be-needed laptop, and I see a lot of used machines offered on eBay with no OS. Does anyone know (not guess or assume please!) where these machines come from? For example, are they likely to be corporate retirements that were purchased without OS in a special deal, or pieces of junk left over after a pirate (illegally) sold off the Windows license?

I want to find out, first, what "no OS" implies about a machine's physical condition; second, whether such a machine is likely to have a valid Windows license, so that I can just replace the disk if there is none and reinstall Windows. (Of course I'll ask the seller before I buy. But if they say "Yeah, it's got a license, no problem," are they probably telling the truth, or almost certainly lying?)

I'm puzzled because as far as I know, all computer manufacturers have agreements with Microsoft that make it impractical, if not impossible, to sell a new computer without an OS. Consequently new machines are almost never sold that way, at least to the general public.

Here are a few numbers that may inform your answer. I searched eBay for Lenovo Thinkpad "No OS" and got about 350 hits, about 200 of which have a disk. "There is no OS because there is no disk" explains only some cases.

"The owner installed Linux" is a satisfyingly logical answer but the numbers do not bear it out. I searched for Lenovo Thinkpad alone and got about 1700 hits; all but 26 specified some form of Windows, and those broke down as: not specified 23, Linux 3. Even allowing for the great number of seller who bungle eBay's feature checklists, that suggests that people who install Linux on laptops are actually quite rare.