If you've been merchanting as long as I have you'll know that items go in out and of fashion, in the sense that for awhile - weeks, months they may make you money - then they'll start to become less and less profitable until you simply are not making that much money of them anymore. This is why a 2010 Runescape Merchanting guide is critical because it means that you can receive up to date information on what to Merchant and why.
I think making money from using armor sets is one of the best and risk free methods of making money around. It takes very little research and very little capital, it does become more effective with the greater amount of gold you have but you can literally start on just a few million and from there you can slowly build yourself up. I'd say it reaches maximum effectiveness after you have several hundred million gold, but if you have around 40 million gp you can get up to 90% of its money making capabilities.
You start by looking at any Barrow's armor sets which are dropping slightly in value, staying similar or rising very slightly. What you want to avoid is any dramatic increases or decrements, as a general rule do not go for any armor sets that have just gone up over 75k gold, and do not go for any which have dropped more than 150k in value. Going for the ones which have gone down slowly is the best thing to do.
You put in an offer for what you want, let's say Ahrim's for example, if its gone down by 50k gold, put in an offer for a set for 50,000 gp under its market value price. See how quickly it buys, if you bought it instantly it means you could have gotten it cheaper, when it comes to the next set lower the price by 25k and keep going in jumps because you should ideally be waiting around 5-15minutes for all of your sets to buy.
Assuming you've bought all your Ahrim's sets for 100k under market value it's time to un wrap them, bank the items and withdrew them noted. When you take the sets apart it means you go around the four hour selling rules which disallows people to buy and then sell items without waiting at least four hours. Since you're technically buying a set, and not the pieces you can sell them whenever you want and so you're speeding things up quite a lot. You will get four items for each group you bought so you should end up with around 40 Ahrim's pieces of equipment. Put offers in the Grand Exchange for one item of each and sell them for 2001gp under average market value. If it sells quickly, raise the price, if after 20 minutes it has not sold at all lower is slightly.
Using this Runescape Merchant Guide 2010 you can easily see that if you sell all forty pieces losing 2k gold each (which is very reasonable - you can probably make money on some of them) you will lose 80k gold, but since you bought them all for 100k under you'll actually be making 920,000 gold pieces!
