Uh... yeah. Buying low and selling high does work. Many have made quite a bit of gold doing this.
And, I personally think that only practice could really teach you this. You just need to know the value of things is all. How low you can buy a thing for and how high you can sell a thing for.
Basically, keep a close eye on people selling rare usually expensive stuff, and sell it on for it's usual price. I've done this a few times, because I'm an evil, greedy criminal mastermind.
