Originally Posted by ImmortalCow

I'll ignore the second half of your question and explain the first half;
The in-store item price is NOT the market price of the items. Some items have never been sold in store.

The "buy low sell high" refers to buying under the mean market price, and selling above the mean market prices. There's two main ways to do this;
A) Buy items that are cheap, and sell them for the mean price: eg if there are 9 Void Forces for 50k each, and 1 for 40k, buy the 40k force and sell it for 50k (or perhaps undercut the market if you want quick profit)
B) Buy items that are cheap or you think will increase in value: eg if there is only 1 Void Set left on the market, and it is only 200k, perhaps you should buy it and hang on to it, as it will increase in value quickly.