How not; wouldn't it be straightforward to have tax rate tables based on transaction size? E.g., transaction <$10=0, <$100=0.1%, <$10k=0.2%, <$100k=0.3%, <$1MM=0.4%, =>$1MM=0.5%, with restructuring payments to avoid taxes to carry heavy penalties.

Not many poor people will be making million-dollar+ transactions, and it seems like taxing ALL transactions will help make the rich pay their fair share, as their money tends to have low velocity relative to consumer goods, but they still do a lot of transactions.

EDIT: Also it should have zero taxes on necessities such as food & non-luxury clothing, similarly to how current state/local sales taxes work.

Also, if there is sufficient tax rate and income to do UBI to above poverty level, wouldn't the need for a progressive tax structure become obsolete?