We have built a trading platform for physical bullion which kind of does the same as that website, as far as I can tell (although that website has a lot more products).
We built the original site in PHP and MySQL, we're currently building a new version using the following technologies:
As you can see, it takes a lot of software to run something like this. That's because there's a lot of real-time background processing involved in this kind of application. Do you have experience with real-time asynchronous processing?
So far we've spent about 2 months developing the new site (the old one was in continual development, I think I've spent almost a full year developing it and updating/ expanding the functionality), and we need another 1,5 months before we can actually launch anything. That's with 2.5 FTEs working on it.
Another important thing is your knowledge. Do you understand securities markets? Do you know the difference between a stop-loss and a regular limit order? What a straddle is? A bear market?
And don't forget security ooh, and maintenance, it's a full-time job for things like this!
As you can see, it's quite difficult to build something like this. Maybe there's software already available for this? I had a quick look on Google, and there's enough to find.