Ahhhhhhhhhhh....Forgive me for being thick, I should have made this realisation earlier. Most people will already have clocked this but I will post this for anyone who hasn't yet. Apologies to noobinator - i can tell this is what you were trying to say all along.
When you buy and sell resources on the Black Market screen, the price for buying is on the left (usually around 450 or so) but the price for selling is on the right (about 250 or so). The left side (quick-buy price) is the MARKET price but the right hand (quick-sell) price is only about 50% of the buy price. So if you sell via this button you basically get only 50% market value cash back.
BUT... If you put your extra resources for sale on the Black Market at the bottom of the screen for people to buy direct, you can name any price you like up to 100% FULL MARKET PRICE. So you get way more cash for your metal and crystal than if you 'QUICK-SELL' it.
So as noobinator says, if you buy low and sell high DIRECT, you can get some serious credit from other players and the computer because is you were to offer your spare resources for 90% market value, that's cheaper than the 'QUICKBUY' for the buyer, and you get 40% more than 'QUICK-SELL' price. Everybody wins, especially you!
All this wasn't immediatly obvious to me but then i'm not a financial person. I hope this helps other new-to-the-game players.