you put how many of the specific resource that you want to sell (in increments of 100). Then next to that number you can increment or decrement the % that you are offering that said resource for on the market. As you hover over those increment/decrement buttons, you'll notice that it tells you what % it is currently set at and if you are the lowest set rate or not. If you are the lowest set rate, then your resources will be bought first and if you're not the lowest then yours won't be bought until you are the lowest.
If you are not the lowest, then you can keep decrementing the % value (decrements by 2) until you are the lowest. If you are already the lowest, then you can increment the % value (increments by 2) until you get to the point where you are the still the lowest by 2% points. If you get all the way to 100% and are still the lowest, then no one else has this resource for sale on the market and you cant ake full advatnage of selling your resources.
In this manner, you can impact what is happening on the black market. Hope this makes sense to you. The easiest way to learn from here is too just play around with it.
Spydyr