I was playing with commodities earlier on the GMAH and noticed something.
The price listed is never the lowest price available, but what I believe is an average of trades.
Anyway, I tried buying a single Tome of Secrets at the 650g price, I noticed if any are lower you get refunded the different and get your item + change for the lower price - price shown.
So, I decided to try something. I listed 100 Tomes at 2g each, then quickly have the search page ready searched for 100 tomes for easy buyout (need to be quick)
I did this about 10 times total.
I tested buying a single tome once my 2g tomes were all gone, and noticed I was now buying them at just 450g a piece after the refund... The lowest I got it was 350g a piece.
Assuming you are quick enough you pretty much get all your money back - the 15% fee, but it heavily disturbs the auction house prices...
What happens now is this -
People who are stupid and go list a stack of tomes and just list it as the automatic price (this price is an average) when I was dragging over tomes it was giving me around 450g as the recommended price, so idiots would just list them and go.
With enough time and resources you could effectively cripple the AH prices and buy things lower than you should.
Maybe people can go have fun with this, test it on a cheap gem or something and see what I mean.