Not sure if the law has changed, but taken from an old ask the warden column;
“It is unlawful to sell, trade or purchase any species of bird or mammal or part thereof found in the wild in California (Fish and Game Code, section 3039[a]). Mule deer and elk are found in the wild in California, so unless his mounts are white-tailed deer or some other species such as caribou or moose that are not found in the wild in California, he would be breaking the law.
Generally, the sale of any (wild) bird or mammal or the parts thereof is prohibited as just stated. However, "shed antlers"or antlers taken from domestically reared animals that have been manufactured into products or handicraft items, or that have been cut into blocks or units which are to be handcrafted or manufactured into those articles may be purchased or sold at any time (FGC, section 3039[c]).
On the other hand, complete antlers, whole heads with antlers, antlers that are mounted for display, or antlers in velvet may not be sold or purchased at any time, except for special conditions authorized for taxidermists (FGC, section 3087).
In the scenario you describe, the antlers appear to be "complete antlers" and would not be legal to sell. If the antlers were "sheds" and either made into products like belt buckles, for example, or cut up into blocks or units for the purpose of manufacturing products, they could be sold.
One other exception, the law does allow for the sale of deer skins or hides of any lawfully taken deer. Deer hides may be sold, purchased, tanned or manufactured into articles for sale. They may also be donated at any time to veterans’ organizations or veterans’ service committees for use by veterans for rehabilitation purposes. Your friend might want to check with his local veterans organizations as he may find they will take the deer hides off his hands.”
Horniac