Where can you buy half priced potions?

I have heard rumors that you can buy health potions and dyes in Diablo 3 for 50% off the normal price. Where can I find these vendors?

After doing some research I found the answer.

Select the quest Trailing the coven - Begin quest.

Now on the way to the spider there can sometimes be a merchant after the bridge.

If not continue to the spider cave, you will notice that you will activate a checkpoint. Now town portal, back and leave the game. Click on Resume game and check the spot again. From my testing its about 25% chance for him to be there. I bought 500 and placed them in my stack.

In the long run with the potions be 305 instead of 610, this is a savings of 152,500 gold.

Tip for saving gold on mystic healing potions: Load Act II 6. Betrayer of the Horadrim:The Forgotten Ruins. Take the Path to Oasis port. In the area right around the entrance there will be one of three cellars, keep reloading until you get the Rotting Cellar. Clear the mobs inside and buy health potions and dyes for half price.