I was hitting my head the other day, out of the game, trying to figure out how the lower part of the Black Market buttons work, and why on earth anyone would want to sell items directly to other players.
So a few trips in game gave me the skinny, so here is what I can gather :
Below the "Sell 100" and "Buy 100" buttons on both the Metal and Crystal side, are two other buttons that have to do with selling to players.
For the Metal side, there is a button on the left that shows your percentage of market value at the time for other players to purchase, and to the right of that is how many of that object (in this case Metal) you have up for grabs to other players. (reverse these instructions for the Crystal side)
The percentage at 100% is the current market "Purchase" value, meaning that anyone who buys from the Black Market will buy from you insted. The percentage at 0% means that is the current market "Sell" value, so if you are using that number (which you shouldnt), you will earn exactly as much money as if you had just quick-sold that item yourself.
So for almost all cases, (except when the market value can go very high and very low, and if your percentage is below 50%), you can sell Metal & Crystal to other players for a larger profit value than you would just quick selling the items to the Market. If you are paitent, you can make their purchase your gain, and if you are TEC you have additional bonuses later in the tech tree as well.
But hold on, thats not all - other players in a game can change the percentage they wish to value their goods for sale at. The player with the lowest percentage value will be bought from first, keep that in mind in large games with many players. (I am not sure if that actually effects market value. I dont think that it does, and that the Black Market just "takes" the rest as a finders fee or some such). So by changing the percentages compared to other players, you can up your chances of having your goods purchased first or more quickly than others.
Also, if you mouse over the percentage buttons, you can see if you hold the lowest percentage in the galaxy at that time, to ensure that your goods will be sold sooner.
You may also remove metal/crystal from "storage" on the Black Market at any time, as long as you have some amount in the "sales" category. This is at no cost at all, so if you are hoping for a big return on other players purchases, and dont get it, you can take back your items at no loss!
So if I put up 200 Metal to sell to other players, and 15min later im in dire need of Metal, I can pull up the Market to see if any of my Metal has sold to other players. If there is still some left, I can remove it and place into my reserves again (using the right click).
Also remember that you may place larger denominations or percentage changes on the market, by holding down the SHIFT key while right/left clicking.
Hope that helps anyone!