As i've said before (in this thread).

I use reduced prices when selling items in bulk, but increase the price for smaller amounts by looking at other sellers and their pricing.

I think for awhile i was selling vitriol (to those with no beast vendor access) at
10 for 800 each (8000)
20 for 650 each (13000)
30 for 500 each (15000)

...coz lets face it, people crafting using that material can afford the price hike >. . .

anyway. people caught on and tanked the prices of that to below vendor price =_=