Uhh, you pay for convenience.
There is a fully functioning CoD system for those of you who want to make the extra buck.
Given the relatively small storage space, along with high velocity of drops, the auction house essentially acts as a secondary bank and selling agent. You get a lot of bang for 10%.
In the longterm, you'll be glad for the 10% tax - it will prevent price floors from crashing. For commoditized items, you'll make more.