Take-offs don't always fit on other guns, I don't bother with used.
I have several unused NOS parts, including critical new hand & new ratchet, for a Victory model in .38 S&W on the way, and I don't even have a Victory.
Yet.

I buy new triggers & hammers and occasional small parts when I can find 'em.
As it stands, I SHOULD be able to keep my Smiths running for my lifetime with "new" factory parts, independent of the factory.
After that, somebody else's problem.

Doing the same for other makers' key guns in the vault.
Except for Ruger, as noted.
Denis