From AS' faq under the "Help tab":
"Max Bid
Your maximum bid will be in the currency of the auction. If the seller is accepting US dollars, then a max bid for 125.75 will be for 125 US dollars and 75 cents. And if a seller is accepting German marks, then a max bid of 75 will be for 75 German marks. There is no need to make conversions. Simply enter your maximum bid in the currency that is being accepted."
You can also see other responses to this question by using the "search" at the top of this page and searching on "currency".
Hope this helps.