a good way to merch in f2p is buying god sets (guthix,zammy.sara) for med and sell parts for max price or doing technical trades like buy an item when it is low and there is a buy signal and sell if they are high
well in the ge. you cant really make a lot of profit. Max profit i made was maybe 10k. (Yews). I think if you could merchant like 10m+ then it'll be good to merchant at GE.
When the price of a object which price fluctuates frequently goes down..you buy it,because you can safetly assume that the price will go up because it has in the past.
I did something of the sort on a zammy skirt..got 7-11k profit?