California is becoming more aggressive by the month and year about collecting use tax on out-of-state purchases. Californians have long considered it a right, I believe, to order from out-of-state to avoid the sales tax they might pay on an in-state purchase, but out-of-state purchases for which state sales tax was not collected should have the use tax paid voluntarily by the purchasers. Many years ago, California provided a line on the Form 540 for taxpayers to self-disclosure their use tax liability:
I find that in spite of informing clients of this responsibility, clients choose not to pay over use tax.
California has a fairly recent publication (a copy of which they mailed to me as a tax preparer): Guide to Reporting Out-of-State Purchases. See the link to this publication in the Tax Links section.