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?

When you say transactions, what are you thinking of? I was thinking of your usual VAT/GST - in that case any sort of progressive tax would be hell for small businesses (or large ones) to implement.

If you’re talking about banks taxing financial transactions, that’s something I haven’t really thought through- it would have some interesting side effects and work arounds.

I like the simplicity of a flat consumption tax combined with a UBI - I’m not sure how politically viable it is, though.

I'm not sure how politically viable it is either — the wealthy owners of congresspeople will scream like they are being murdered.

I wasn't thinking of any kind of VAT, where you need to subtract out your cost, etc.; that is a mess! Just a straight shaving off the transaction. The idea is to spread it over so many transactions the rates are so low it isn't worth bothering to work around.

For example, if you try to structure a $1million house purchase into ten %$99,999 payments to get from 0.5% down to 0.4% rate, you will save a grand total of $1000, hardly worth it considering there should also be laws with strong penalties for such structuring.