hii
merchanting will be a lot more difficult these days with the new trading cap limit. so buying low and selling high won't work unless you're charging 3k above the market price.
meaning you buy a dskirt, the market price for example is 500k for the skirt. you can sell it tops - 503k. UNLESS the person you are trading with, and yourself, have got enough quest points. the higher the quest points, the more you can profit/lose. i can profit/lose around 16k every 15 minutes. a new player can only profit/lose 3k as they have 0 quest points.
BUT.. you can still merchant. buying things on the low range on the grand exchange, and selling them on the high range. sometimes people will want to quickly sell stuff, and sell it at the lowest to get it sold right away.
the best things to merchant these days are raw goods that is often used. such as yew and magic logs, or coal. buy at the lowest, sell at the highest.
although pk'ing in the wilderness is gone, people still will need food. for bounty hunter/clan wars, and also for fighter monsters. now that pk'ing in the wild is gone, more and more people will start killing high leveled monsters for drops/profit, and they will need food to heal.
you may notice you aren't getting as much gold when selling the food. but at the same time, i'm sure you're buying it raw cheaper than before (unless that is, you fish it). prices may have dropped on a lot of things, but think of it this way:
the things YOU need are going to be cheaper for you as well. so it kind of evens itself out in a way.
good luck :)