You want to merch items that RS gold show a steady change, if that makes sense. Things that increase and decrease often, but is predictable. To see when things will be able to rise or fall out of this cycle, check for big updates , like days with double xp or new quests. Try to merch items that most people will use for skilling, and do it in bulk.
When buying on the exchange, I typically go around 5%-10% lower than medium cost. You can then decide to sell it once the price rises for about 5% to 10% more, or you can determine the price you want to set for your "wholesale" price, which will generally draw more attention to selling outside the g.e.
If it doesn't purchase or sell immediately, don't be concerned. If it's not a purchase then it's likely because you're buying at a low price, and only those who aren't concerned or looking for cash can sell for medium prices. Generallyspeaking, the cheap Runescape gold item will buy by the end of the day.