First, you have to purchase a shop at the Market. You can have upto 4 shops in any settlement, including those belonging to your enemies. There are 5 slots for your goods in a shop. You can trade anything that can be in the inventory: artifacts, resources, magic spells, items, units... The process of passing items to/from the shop is identical to any other transfer of items in the game. You can set the prices for your items when you enter the Market. Items with prices different from 0.00 immediately become available for purchase by other players.
A newly purchased shop will be at the bottom of the trading shops list. The delivery speed of the goods is 2 hours per cell.
To increase the position of your shop on the trading list, it must be upgraded: the higher the level of the shop, the higher position it will occupy on the trading list. Low positions do not generally attract many customers.