It's mostly experience with the market that gets you used to the prices.
I myself know which items sell well, and for how much I can buy them for, and then resell them for. With experience you'll also find out which type of items sell faster, and why.

You can't really teach someone to market, they have to find out what exactly is going on by themselves. You can however teach the ways of haggling people to get a lower price for the items, then what you would actually pay.