For example, the Apollo 9 CM here in San Diego is missing all its DSKYs...
When did that happen? I was inside that CM when it was in Dearborn, and the control panels and LEB were intact, although the guidance platform behind the LEB had been removed for separate display elsewhere.
...but here they're available on the market. Hmm.-
My old boss has a spare DSKY from Apollo 14 in his office. He retired long ago, so I need to hit him up to see what happened to it. I'd like to see if he'd bequeath it to me, whereupon I would put it on permanent display at Clark Planetarium here in Salt Lake City. In his case he merits one, because he worked on Apollo 14 and was one of the engineers they dragged out of bed to help solve the docking problem.
but how does flown NASA hardware legally make it to the private market?
Indeed. Technically flown hardware is still government property and must be disposed of through proper methods to avoid insider profiteering. Unflown hardware that was never transferred to government ownership is the property of its original contractors and can be disposed of in a different way.
I own flown SRB hardware. But it has been officially discarded by the government; they're SRB components that are not reused and are supposed to be replaced with new components during refurbishment. So those parts, when removed during refurbishment by Thiokol, revert to Thiokol's ownership. Since the parts I have are made of Inconel, they are almost all recycled metallurgically. But a few are retained to be given away as "flown hardware" souvenirs to Thiokol
preferiti.