As MarvinDaMart said:
Oh yeah its the easiest way to make isk. But don't kid yourself that your performance at the start can be extrapolated to what you make when you have 10s of billions of isk. As you start to saturate your markets you find that new isk markets become harder to exploit to the same degree and rather than 18% / 4 days you may find that its closer to 8 or 9 %. I currently average 1.5% per day but this is on tens of billions of isk so that amounts to hundreds of millions / day.The point is, every market has a sweet spot where beyond that the more you invest the lower the return. You can't buy more items than people are selling and you can't sell more items than people are willing to buy. Plus, competition from other traders cuts margins, as do game mechanic changes, market fluctuations, etc.
BTW you should check out the eve program called EMMA on the forums. Its a great way to track your isk making progress. It cost 100 mill but its well worth it imho. Anyways good luck!
My last post should have had a disclaimer about these facts but I was a little too "in the moment" while writing it, drooling at the thought of buy a few dozen Wyverns for use (ignoring the fact you can't dock them). ;)
Still, there is money to be made by the patient and the clever. I recommend giving it a try.