I like to see the bid prices on the post tab (activated by "/aux post bid") and noticed that the bid prices for existing auctions are wrong sometimes. Either it displays the stack bid price, even though it should show the price per item, and sometimes it displays a bid price that is just very wrong and can't be found if you search for the same item in the search tab.

Some wrong prices I can't reconstruct, but I noticed that if the bid prices are way too low, they sometimes correspond to a real bid price if I multiply them by the number of that item in my bags. So somehow, the quantity of an item in your bags is part of the calculation of the bid prices of existing auctions in the post tab.

Hope it's clear what I mean, I can post pictures otherwise.

It's meant to be that way. It shows the stack price divided by the stack size you've selected for your auction.
The listing is meant to help you undercut the stack prices of other auctions with your own stack price (because the default auction house lets you sort by stack bid price only, so you will appear higher up in the listing)

Basically it's a listing of stack prices, sorted in ascending order by stack price. Just the prices are normalized with your own stack size because it's more useful for comparison because the whole rest of the post tab is based on unit prices.