Did someone say my name?
Good thing I keep my watches either in my collection, or with a few trusted folks here on the forum Folks have asked me why I never switched my El Primero rotors out for gen 4030 rotors, or red reversing wheels, etc. The short answer is that my intention is to never pass these off as 100% genuine (only partially gen ).
Since these things are built solely for personal enjoyment/challenge, and since I know what's behind the closed caseback, I've always felt that spending the money for those 'final touch' movement parts was kinda pointless; better to spend that money elsewhere, or on other projects
More on topic- The best way to ensure the legitimacy and authenticity of a second hand purchase is to buy from someone reputible, or buy new from the AD. Granted, nothing in life is guaranteed except for death and taxes, this will help mitigate the potential risk of buying something bogus.