California prohibits private transfers. Firearms sales between private individuals must pass through an FFL and the appropriate background checks must be done on the firearm and buyer.
Idaho doesn't inflict that sort of BS on private party transfers. They also do a lousy job of tracking violent sex offenders, but that's a different problem. Having a CCW in Idaho is compensation for lack of reasonable care in some other areas.
I'll assume that people who actually believe that kind on nonsense will only buy or sell cars through licensed car dealers for the same reasons. Even if they do, these cowards still benefit indirectly from private transactions they don't participate in. Direct private transactions keep wholesale and retail prices closer together than they would be if all transactions were mandated to go through third parties. Just imagine what you would get payed for a used car verses what you have to pay for one if you had to go through used car dealers. If private transactions for any good are mandated to go through only licensed dealers for that commodity, the market will become less efficient, net commerce in that good will decline, and consumers will loose out to the exploitive benefit of the dealers.
The government will also be able to collect them more easily.