Consider: Someone posts 522 crafted gear on the AH, but the buyout is 2000g instead of 20,000g like a few of the others (as an example). You thank your lucky stars and you buy the item for 10% of its actual value. You suddenly get a message from the seller, who begs you to return them because he missed a "0" in the buyout field. Do you:
a) Give them back.
b) Tell them 'too late, that's the price you set, that's what I paid for'.