you just have to learn, back when I was doing hell I had no idea, now I know prices for almost every drop I get, its just something you pick up after trading a lot
I wouldnt worry too much about maximising profits up until you are farming inferno items. You might lose a few thousand here and there now, but if the items are selling and you have enough to buy some cheap upgrades you are doing ok.
Try to buy really cheap upgrades though if you need them because once you hit 60 you will want to have some gold left to buy nice level 60 items
Edit: Thought Id add that when you hit 60 you will learn pretty quickly which items are worth selling and which should be vendored/salvaged. At that point it becomes worth having a quick search to see how much competing items are selling for then undercutting.