You can legally sell the kits, but they can only be used to rebuild magazines that you owned prior to the ban and can only be used in guns that you also had prior to the ban. You cannot legally insert a magazine that is able to accept more than 10 rounds into a firearm that you acquired or arrived in CA after the ban.
You can have all of the high-capacity magazines that you want, provided that you owned them (all together now!) prior to the ban being enacted. Do you have to provide proof? That depends. There are no date stamps or other markings on any mags that I am aware of that show when they were made. You might get into a tight spot if you are on your way to/from the range with a post-ban firearm and a pre-ban hi-cap mag. The cops might demand that you provide proof that you owned them pre-ban and may even detain you. But as long as you do not insert one into your post-ban firearm there is technically nothing they can do to you. Of course you might spend a few nights in jail before they figure that out but it is a price you may have to pay.
Many gun owners will store their pre-ban magazines loaded next to the gun safe to save room
, but never insert them into a rifle (except in case of a Constitutional emergency. Then all bets are off.)