Can't sell stacks

When you go to sell a stack of say Scroll of wisdom you only get the 3gp for the whole stack it does not count all 40x3. That is what I have found. I tired to sell my extra orbs of Augmentation had a whole stack of 30 and only made 30 gp off all of them. I tested this again to see if I made a mistake with just 3 and still got 30 gp instead of the 90 it should have been. Since each one sells for 30 gp.