It's pretty much economics.
The black market deals a lot with scarcity and abundance. If a lot of people are buying crystal, the price will go up because crystal will get more "scarce" in the market. Likewise, if a lot of people sell crystal to the market, the price will go down because there's so much extra crystal. "Flooding the market" occurs when a lot of resources move at once. If a lot leave the market, it will "boom" for that resource, and the price will skyrocket. If a lot enter the market, the market will crash and... well, you get the idea.
The "total offer value" buttons let you sell your resources to another player directly. Prices are still tied to the market, but you control them (somewhat). Here's an example of how it works:
1) you put 100 resources on market at 50% of market value.
2) Price in the market shoots to 500 Credits to 100 of resource, but YOUR price is half that.
3) Smart people buy your cheaper resource, and you get cash for your surplus resources!
Of course, it's probably not wise to have the price at 50%, unless someone is offering a lower price. The person who has the lowest price has his/her resources sold first.
Sorry about the length... I hope this helped!