Flipping is taking advantage of another player's impatience. For example, you may buy 10 pairs of dragon boots from people who've just come back from Godwars/Cleaning their bank. These players tend to sell them for a low price, and you can easily snap these great deals up. Then, 8 hours later you can leave an offer in the ge selling them for 10-20k higher than you bought them. The same process applies here; another player might want them for dragon-slaying, or perhaps a quest. These players will buy for higher amounts, and you will make PROFIT.
There's plenty of items you can flip with but the item must be traded in high volumes each day.