Publicani

The Publicani were the Tax Collectors under the Roman Republic (509-27 BCE).
They consisted of Joint Stock Corporations (Societas Publicanorum or Soc. Pub.) formed to create Tax Collection Companies.

Rome would put up for Auction the Taxes of a particular city every few years.
The Publicani would bid for it, and the successful bidder would pay the Roman Treasury in advance.
As this acted as an advance loan, the Publicani would be paid interest on it by the Treasury at a later date.
The Publicani then had to convert the Taxes they had gathered, whether in Cash or Goods, into coin.
If they took more than they bid, they could keep the surplus, plus the interest due from the Treasury.
The Publicani would then aim to collect the Price Bid, plus the Interest due on the loan. Anything surplus to this they could keep.
In other words, the Publicani took the risk that they might not collect enough Taxes.

Vectigalia and Portoria through the Portitores (officials who collected the Portoria).

To avoid losses, the Publicani often formed an alliance with the Provincial magistrates and Farmers and bought large amounts of grain in advance at a reduced price, and then held these as reserves.
The Publicani were also in a position to lend money to Province Officials with rates that could vary from up to 4% per month.

Corruption under the Publicani had made many immensely wealthy. But after they had bankrupted the Provinces through loans at extortionate interest rates, Augustus decided this could not continue.
Augustus abolished the collection of Taxes by the Publicani, and collected the Taxes directly instead through the Procurator of each Province.
The Publicani continued to operate as Bankers, but no longer as Tax Collectors.
The official Roman Bankers were called the Argentarii.

To achieve a uniform tax collection system, it was necessary to assess the wealth of each Province through a Roman Census applied every 14 years.
Property was no longer the sole criteria.
Instead, a wealth Tax on each city was imposed of 1% and also a Poll Tax on each individual at a fixed rate.
Taxation now moved from being based more on Income, than being based on Property ownership.

The result was that the Tax Revenue varied from good years to bad years, but was considered fair, compared to the previous system.
However, new Taxes had to be devised, either because the costs of running the Empire had risen or because the Tax Revenues had fallen compared to the system of using the Publicani.
This became increasingly an issue throughout the Empire as existing Taxes were increased or new Taxes applied to balance the Budget.