all of the lower lv stuff u will get the same at both but if you sell higher lv stuff at market you can get more money
  if that makes sence to you

Store: Sell it for 50% of real price. 
Market: If it is for example, a rare and powerful weapon,  some people will pay up to 3000% for it. So if you get a weapon you know some needs put it at the market. Say you sell it at 200%. Instead of making 50 silvers, you make 200.