Prohibited to import into the state in its current form. They can be permanently blocked to hold 10 rounds only but this must be done by you first before shipping or the buyer can elect to have them sent to a middle man who will permanently block them and then ship them into California.
You can no longer send the magazines disassembled as parts into California, so if the buyer asks, its a no go.

Are the magazines you are selling rare or are they an uncommon magazine?? There are a lot of places which already sell blocked mags to California residents so that a middle man isnt needed.

Yes, if we already own full capacity magazines then its grandfathered in and we can use them but we cant import any into the state at this point in time unless they are perinatally altered to 10 rounds only.

LEO can have full capacity magazines but it usually requires a letter from their office and its recommended that they be shipped to an FFL or directly to their station for verification.