Exchanges between merchants and their patrons could be a little complicated because the merchant could rip off the patron and leave no proof. However, several nations forced their merchants to issue bills of exchange after every sale, otherwise known as receipts. This simplified the system of buying and selling and provided protection against dishonest merchants. In the game, it instead reduces the fee associated with buying and selling resources to 15%.
In practical terms, a reduction of the trading fee means you buy a resource for less, sell it for more, and the price changes by a smaller amount after every transaction. The standard fee is 20%. Bills of Exchange reduces the fee to 15%. Some civs have policy decisions that knock it down to 10%, while Poles are apparently very good traders - their markets always charge only 5%!
Researched at: Market
Cost: 300 Wood, 200 Florins
Effect: Trading fee reduced to 15%.