I recently bought a plan for Exalted Fine Dread Shield off the Auction House. The buyout price was 30k gold for the plan and it literally took me 2 seconds to click buyout but the plan's price went up to 395k in that little time so I bought it for 395k unknowingly.

Now, AH normally sends your money back if you are outbid or tells you that you don't have enough funds for purchasing the item. I clearly had enough funds but I didn't know -and never thought- the price could go up that fast and that much. Also after I clicked buyout the "are you sure?" screen showed 30k as the buyout price but again 395k was gone. I think you should check this and make the user unable to buy -until a new search is done- to buy the item if the price has changed.