To merchant items in Runescape, you really don't need much except some initial wealth. The more money you have to begin with, the more you will be able to earn. With more money, you can buy more of the merchanted items, giving you the opportunity to earn more money. With less money, you may only be able to buy a couple items, giving you a smaller amount of opportunity to earn money. With that said, you should try to earn enough to buy at least 10 of the merchanted items to really be able to make it worth your while. This could be anywhere from 4 million gp to 20 million gp, but don't limit yourself if you have the ability to earn more money, as more is better to earn more with merchanting.
One last issue you might be wondering is when should you sell your items? Sometimes the clan will tell you when to sell, but by that time the clan leaders have probably already sold their items and you are going to be up against all the other people trying to sell all of their 
items. The only way to come out ahead, without getting stuck with the items and feeling a loss after the price starts to drop before you can sell your items, is to sell when you feel satisfied.
Personally, I have waited for clan leaders to announce to sell and the price ends up dropping by half before I can even get rid of the items. Once you feel you have earned enough from that item being merchanted, sell them! Come out feeling ahead, not like you've lost because you couldn't sell at the highest price possible. Doing this will keep you wanting to merchant more items, and also keep you from potentially losing money.