I had to sell my old euro car (to an out-of-stater) when I moved to CA because despite miraculously meeting strict CA smog guidelines it never had to meet in its decades of existence, it had non-approved (i.e. $CA stamped$) parts in it and failed the "visual". Clean-blowing, perfectly functional car, had to sell it anyway. I'm still bitter.

you have to prep carefully for the visual...

>What are you stuck with besides standard registration?

I've never done one for CA but some states make it hell to go from "car with no paperwork last owned by a dead person and last registered years ago" to "car with proper paperwork to sell/register" so for someone repairing old cars to flip that can cause a significant headache.

I literally get cars from junkyards with no paperwork. Example: a wrecked, rare 76 Jaguar xj6C coupe with no paperwork, plates, keys. You have to trailer it to the police station for verification, show bill of sale etc etc, then go through months of DMV forms and fees which typically require a professional who does this bureaucracy, otherwise you will spend tens of hours waiting to go to a DMV window to be given the next hurdles to jump over. Transferring titles from purchased out of state cars is painful too. The smog issues are relatively straight forward in comparison.