Say you have 10 of the same item that you want to sell. I just picked something I had in my bag (some dye), went to the “Sell” tab and right clicked the stack of dye in my bag and it was into the Sell tab like this:
The AH automatically offers it at a price that matches any other sales of that item currently on the AH. If there were no other offers for that item it would say “No items found” on the right hand screen and put a minimal amount in the “Unit Price” box which you would then need to change to what you think would be a reasonable price per unit. If you dont use any auction add ons you can go to a site called The Undermine Journal, pick your server and when you get there type the item into the Search field to get an idea of what it has sold for before on your server.
You also need to pick the duration of your sale - the longer you put it up for, the higher the AH fee is. If you think it will sell fast, pick 12 hours (though obviously not a good idea to do tht late at night as most of your AH time will be used up in sleepy time).
Im not sure why an item you list as shown in my image would sell at the unit price per stack unless you failed to change an item price for something that the AH could not suggest an actual true value for. As long as you have a unit price set and a multiple quantity listed, it will sell at the Total Prices shown on the screen.
Of course a complete stack will not always sell, sometimes only parts of it. Somebody might only buy 3 of my dyes so I only get 3 x unit price.