I sell products with the highest buy limit (10k-25k). I tend to RuneScape gold flip. I concentrate on foods as well as runes and pots. I keep track of a few things which i am sure crash occasionally and earn profit from it too. I'm always changing the expensive weapons and armours that i utilize to save my money and earn more occasionally. I sold my bgs prior to it crashed at 8 miles then sold my fury shortly before it crashed, then purchased one for 13 mil , and was able to sell it for 14.5 mil. The claws were purchased, lent to friends for 1 mil and the rest was sold. Since armour and weaponry are often in a perpetual downward spiral, it's good to have a plan like that.
I'm not able flip barrows, any other weapon or armour for any reason very well. Tips? I'll buy almost all items that are trying to sell quickly and make a profit from it. For example, my fury. I also earned 150k from a person rush selling his whip, for one mil. It's like that.
My time of flipping depends on the item's trend. If the price is rising the maximum price I'll purchase at the min price (how I decide on max and min prices will be discussed below) before selling them at their highest price as soon as the transaction is finished. Sometimes, i purchase/sell at 1gp or more over the min/max, depending upon the margin of profit and how quickly i can tell them to buy or sell.
If the item's price is falling, I'll purchase for the minimum price and immediately sell it at the maximum price while still purchasing at the minimum price. My reason is that if i wait the sometimes 10 mins to purchase a product that is declining, the maximum will decrease in the time.
Now how i determine the "true" maximum and minimum. I purchase 10 of an item far more than what it is worth and then sell it for far lower than what it actually costs. The program will pay me back if I purchase significantly more. It will also inform me the amount it bought for. If I offer it for cheap OSRS gold much lower, it will provide me more than I had originally asked for, since that's the current min price.