If something like <= half the taxes were donated for in-game event rewards or a lottery for new players or some such shenanigans then I'd be cool with this. Why not.
I use the AH a lot so I wouldn't want to see it ruined by taxes either, but if some of the money went towards events then that would be more interesting. That way it's not just a pointless money sink and the taxes ultimately serve to support the game and encourage activity and help new players etc.
Edit - 10% seems a little high maybe, I don't like the sound of losing 100k off a 1million gp sale unless the gp is going towards some bomb ass events. I don't think we need a money sink that severe. 5% or less ought to be enough, you don't want to discourage people from using the AH altogether or the economy will suffer. There's nothing wrong with a bit of free market competition and the AH provides that. It keeps prices more consistent and fairer in the long run just like real life. People are only gonna pay what they can afford and/or what they think is a reasonable price, so prices will always find an equilibrium.
Except for whoever is selling 19 santa hats on the AH and keeps undercutting my price for weeks now. Screw that guy. Upsetting my equilibrium lol. God damn monopolists
But I digress.. TLDR: +1 to charity taxes, -1 to money sink taxes
Peace over anger.
Honour over hate.
Strength over fear.
aka. punKrockeR