When buying on RuneScape gold the market, I generally go approximately 5%-10% lower than moderate price. After that you can sell it once the purchase price climbs for 5%-10% over, or you can think of your "wholesale" price, which will draw more attention from selling outside the g.e.
If something doesn't buy/sell right away, don't be worried. If it does not buy, it's because you're purchasing under moderate price, and only individuals who do not care/are desperate for cash sell under medium. If it does not sell, either it is a product that is not in high demand, which usually means you've chosen a poor thing, or so the price you've set is too large. I wouldn't recommend taking the price down before a day or two once you've put the offer up. Remember, merching does not happen over night; it take alot of patience.
If you are using raw materials such as raw fish, logs, herbs, etc. I would sell on the road for a set price per item. If you are selling anything else (essentially non-skilling items) then I'd say sell it around the g.e.. It may take a while, but this is the best that you can do, as it is harder to find someone looking for ten dragon platelegs than 2k nature runes.
Some general advice: don't be worried about losing money in the beginning. It happens to most of us. When I first began, I lost about 6m about the first day. Just keep with it and you'll come to comprehend the market and economy of runescape better. I've stopped merching in favour of skilling, but it is a terrific, time-saving approach to earn good money. Good luck! Well, in Level 74 I have 200 quest points, and I have been questing since level 15.
Materials to always keep near. Anyone who goes ahead. Well it's always great to have a great source of different potions and other materials like ropes. Having a principal set of combat equipment buy School RuneScape Gold of every type is great as well. Such as having dragon conceal for supervisors who have strong mage attacks, and mage gear and runes for if you need it during certain quests.