There is a great Wiki on this site that I highly recommend you look at http://wiki.calgunsfoundation.org/Bu..._in_California
If the firearm is not on the California DOJ approved list then you need to do a face to face PPL at his or your FFL. If the firearm is on the DOJ list then you can have him send the firearm from his FFL to your FFL. Sometimes an FFL will accept a firearm mailed directly from a private seller (on the California DOJ list) but that is up to your FFL.
Just because the seller is a LEO does not give them the ability to sell you a firearm that is not on the DOJ roster by shipping it to your FFL. Must be face to face. If you are a LEO then that is a different story.
At this point you really need to find a local FFL that you want to work with and who can tell you how to handle these transactions. I have one local to me in Cameron Park that I have worked with several times and has been great to work with. It is their license on the line so you can be certain they are going to know and follow the rules.