"For purchases made out-of-state, states already have a mechanism for collection: use tax. I find it a bit disingenuous that he doesn't even mention this."

Mention: "In California, the State Board of Equalization estimates annual tax revenue losses of $1.085 billion from unpaid 'use taxes,' which are supposed to be paid for out-of-state purchases."

But you're right, he spends very little text on this idea.