For many who don't have an automated tax return it could be easily. It amazes me the systems like bizum and bank transfers don't have a checkbox for "this ought to be taxed".

If you are not getting a VAT refund then you are paying VAT and it is in your interest that everyone does, so most consumers would select the box. The transfer is recorded, consumers would be complicit in the fraud if they didn't tick the box and that's enough excuse for both sides to want to play ball.