Note that Amazon has to charge state sales tax these days too, as do many other online retailers. So "buying out of state" is going to be pretty hard. People's eBay or Craigslist game might get interesting though with the new margins.
As much as California is one of the few states that might be progressive enough to attempt this, it'd be far better if a less populated (and smaller) state attempted UBI first. If Montana torches it's economy, it's probably a lot easier to reassemble than California's.