Olympia: The Age of Gods PBEM (May 4 1999)
The ruler of a city may place a tax on all sales transacted in the city's market and the market in the city's Trading Guild (if any). This tax is a percentage of the sale price of a good, and is taken from the amount paid by the buyer to the seller. For example, if a noble sells 10 iron at 20 gold each, and the city has a 10% tax, he will receive 180 gold, the other 10% of the 200 gold sale price being placed into the local garrison.
The tax on a market (if any) is shown in the market report.
Needless to say, all nobles in the ruler's faction are exempt from sales tax.