EU-Turalyon(A), multiple others.
Hey there. First of all, welcome back to the game.
I'm not sure what is the best way of explaining is, but in my mind I see flipping as a grayscale. On one end there are "white" items. They sell fast, but the profit you make off them are low. A good example of a "white" item would be buying out cheap gems and enchants when they are at or below crafting cost and sell them when prices go up when the demand is higher (raids resetting, popular raid nights). The investment is low, the profit per item is low, profit margins can be 100-500%, which is "ok" and most importantly; waiting times are low.
The other end of the spectrum are "black" items. These are items like top transmog / recipes (possibly no longer obtainable) or TCG items that you could buy cheap on the AH and re-sell with crazy profits. The investment can be low or high, depending on your luck and liquid gold available, profit per item is very high, profit margin depending in your investment but the waiting times are high; it can literally take years to sell an item.
I've mentioned two extremes, but of course there are flipping opportunities anywhere between them, hence the grayscale.
Now with some numbers, I like numbers, the first one is randomly made up but should be accurate,the others are are based on my personal sales. On Wednesdays I try and reset a lot of gems for obvious reasons. If I find out my best selling gem is at 50g/gem, I buy out all of them. I spend 750g buying the 15 gems posted on the AH and repost them at 400g. After being undercut and reposting I manage to sell 15 gems of that cut at an average price of 150g, or 2250g in total. It's an easy 1500g profit that day, doubling my gold invested and triple the amount of gold spend per item. However, I am probably only able to do this once per week at most, meaning 214g profit per day.
Now for one my best flips, I paid 5g for a transmog item, I managed to sell it for over 210k. The initial investment was low, profit margin insane but it took me 8 months to sell; still well over 800g profit pet day. I've also flipped a spectral tiger with a 120k profit. It took me 3 months to sell if I remember correctly, 1300g profit/day, but the initial investment was much higher and percentage wise the margin was much lower compared to my other flips.
What I'm trying to say is that there are many items worth flipping if you have a. knowledge of the game and your server for quick flips. b. liquid gold available. c. patience, this is the key to successful flipping.