From this guide to Empire of Sin, you will learn how to trade on the black market.
You can access the black market panel using the icon located in the left corner of the screen. When you click on it, a special panel for trading will be launched.
This menu opens in purchase mode by default. To change it, you need to click the dedicated switch button. That's when you can start selling your goods. Other than that, both tabs look identical. In there you will find a list of items owned by you or the dealer  (along with their prices), divided into 6 categories . Next to it, you will find the statistics of a given item .
The individual objects were color -coded by their rarity and power. The better an item is, the greater its value. From the bottom, the hierarchy goes - transparent, blue, green, purple.
If you sell items acquired during combat, you can do so without any restrictions. However, if you are buying stuff, you need to keep in mind that the dealer has limited stock, which gets replaced every few days.