I understand the legality of 10rd magazines. BUT, can I sell/ship 7rd mags to an address in Cali?

You may.

What about 30 round mags?

If they're exempt from magazine capacity laws, you can ship direct to them. If not, send to an FFL (which will have a large capacity magazine permit to legally cover the importation into the state)

What makes a particular magazine "exempt from magazine capacity laws?" Date of manufacture?

@leelaw

The person receiving them. I should have clarified.

The magazines aren't illegal, so much as there are laws on who can legally possess them.  There are legal possessors, like LEOs and licensed dealers, manufacturers, etc., and prohibited persons, like felons who aren't supposed to have them at all.   For everybody else and perhaps millions of mags, they were initially not regulated, then "banned" but possession was grandfathered, then for a week during litigation new acquisition was allowed, and then back to the the original possessions and newly possessed mags being legal to possess.

Since most of the mags aren't dated, enforcement is almost impossible although they get into the news now and then, usually as a part of some greater incident and arrest.