That webpage gives a very good start to the process, so I won't re-hash it.
But I'll try to add what I know.
To start with:
when you're a novice, check out both sides before you buy, all 3 pits are close enough
don't try and make a killing in one commodity, on one short run. It's far too easy for it to get ruined by anyone else. (or your own purchase, for that matter, which DOES affect pricing)
avoid selling to 'Good Stores', unless you know exactly how many you can sell
the best deals come from the buy side, NOT the sell side. It is amazingly easy to push down an inflated price. So look for underpriced commods to buy, rather than overpriced commods to sell. Occasionally, you WILL get trumped by the system, or by another trader. It's far better to warehouse underpriced commods that you can sell later at average, for a profit, then to have to sit on the commod, waiting for inflated prices to return
don't assume logic should dictate how the system works, just get to know it
generally speaking, keep your purchases low, and spread them around - the system 'knows' what you are doing, and makes adjustments, plus... you can easily mess up everybody else by unloading a bucketful of one commod
the warehouse is your friend- and not just for saving your ass... you can easily make money by not even being in the game! store underpriced commods, wait for them to go back up to normal levels, and sell...
lastly, and most unfortunately... the game itself won't help you make money. Usually, it'll help you lose it! You need to keep track of stuff, on paper, spreadsheets, whatever. If you don't like that kind of thing, then don't bother with commods.
that's all I can think of, off the top of my head
Marcomir