Depends on if you mean RP or general survival.
For general survival, the more you can sell for less, the better you are. Also take note on what everyone sells. If you jump into the god armor business, expect to have very low prices due to the inflation. Same goes for ores.
I'd probably suggest looking at what items are used most by new players, and then what builders like to use. Find the prices in the current market, and lower yours to be less than them.
But #1 for any market is advertising. If you can bring people in to look at items, chances are you will make a purchase.
And for RP, pick something nobody else is filling, like bakers and farmers. You could also join the Merchant's Guild and talk to other merchants.