To say that California's gun laws are strange is well known.
For a FFL to ship into California, they have to register with the State, doesn't matter that the FFL already has a Federal License, and then get a Firearms Shipment Approval number from DOJ, for any gun shipped. 3 guns, 3 letters. It is automated and easy but, my view, it is purely there to frustrate the 'import' of guns into the State.
If it is On Roster, is not banned outright, has the wrong dash or feature then all is good; if not, then the fun begins - all between the FFL's.
Yes, it is up to the buyer to make sure of these things but I am sure that a seller, once burned, will just say "no sale to California" to make their life easier.
There is more to this but space is short and time is valuable; just know that life behind the wire is complicated for gun owners.
As an FFL, I've jumped through the hoops to ship to CA a couple of times, as stated above, it is PITA. Doubt I'd do it again.