I'll add one more point of explanation:
Some states force all private sales to go through an FFL. The FFL doesn't do this for free, and the fee reduces the net amount received by the seller. It can exceed the value of the firearm.
Some propose to only close this alleged "loophole" at gun shows, leaving person-to-person private sales unregulated. But, it doesn't account for the reason that many people prefer to conduct their personal sales at a gunshow: security.
It is difficult to safely sell a firearm, because you have to advertise it for sale, making the seller a target for theft or burglary. And, there aren't safe places to meet for an exchange, either. My local police department offers their lobby as a place for people to exchange items they have listed on eBay, etc.... but they explicitly prohibit guns, knifes, etc. because the location is one of the few places where possession of a weapon is prohibited by law.
In California the fee is added as a surcharge to the total. The problem is the fee for handling a private party transaction (PPT) is limited to $10 for the FFL, who may lose money on the deal, which includes storing the firearm for the 10 day waiting period. The law requires a FFL to do a PPT but many of them try to put impediments to such transactions in hope of influencing buyers to go elsewhere. It is not unheard of for a PPT to take all afternoon as the FFL stops working on it to serve regular customers. Like everything else about CA's gun laws it is all about discouraging gun buyers.