Once you have determined that the gun is legal in California, you can complete the purchase with the seller. This usually involves sending them a check, money order or credit card information to complete the transaction. They will have to ship the gun to a Federal Firearms Dealer in your area to transfer the gun into your possession. The FFL dealer will have to FAX a copy of his license to the shipping dealer before they will ship the gun. You will need to provide the FAX number to your dealer or if the shipper does not have a FAX, your dealer will need a mailing address of the shipper so a copy of his FFL can be mailed to the shipper.
If you are purchasing the gun from a private individual and it is merely being shipped by a dealer, I will not collect sales tax if the private party seller will include a copy of the sales receipt showing the purchase price, the model of the firearm with serial number, a statement that he is selling the gun to you, that he is not a dealer, and listing his name and address.