How much are you willing to put in/invest? Take a look at the GE graphs for certain items. Knowing your own merch is far safer opposed to jumping on the bandwaggon of known merch/flips, as it generally results in loss due to people dumping early and what not.
Look at the G.E graphs! When you see an item that you think is about to rise mass buy that item wait a couple days (sometimes weeks, takes patience) and when that item has risen enough sell it for profit!
Beware - merching can become very risky and you will most definatley lose some money at one point, but be persistent and no doubt you'll make sums upwards of 100M!