No, it's not too late to do this correctly
The buyer sends his FAC to the seller who completes table 1 of the buyers certificate and then sends the certificate back to the buyer.
The receiving RFD holds onto the rifle until the buyer appears with his certificate showing he has authority to acquire and that the seller has correctly completed table 1 of the buyers certificate.
The seller informs his local police of the sale, giving them the details of the buyer (not of either RFD)
Cheers
Bruce