From start to finish, a brief summary as I understand it:
Difficulty Selection: Harder difficulties will give you smaller windows to time your clicks but yield better prices for you if you succeed.
Haggling: After selecting difficulty, you are brought to a horizontal meter, an arrow will move side to side constantly after you begin. You wanna left click when you are within the borders of one of the icons on the meter, successfully timed clicks give you better prices, missed clicks give you worse prices.
Haggling ends Automatically when you reach the best discount.
You can run out of icons if you take too long, if its going badly, try and get a few in a row and end it manually to break even.
The Green Icons at each end of the bar end the haggling manually when clicked.
I have the Hard haggling unlocked and there are no bad icons to click on, so click em all! (may be different on very hard).
Hope this helps!