Question about NPC traders

I'm curious as to how many of you guys are having the selling price problems at the NPC traders (for example, for me, the rune trader pays 25g for a major vigor). I know I'm definately not the only one, but a few have said they have no problems, or that their problems are intermittent..

The traders work on supply and demand basis, if quite a few people have sold major vigors to the NPC then the price will drop to 25g, however if people start buying them from the NPC the price will increase again.

It's the same as dyes, a black dye from the trader can cost as little as 3k on occasions and over 5k on others, theoretically you could make money just by buying and selling items back to the same trader, just wait for the price fluctuations to happen.