From velhin:

Is there a way you can buy multiple, for example; if I want to buy 60 Felblight, do I have to count out that I have bought 60...or can i tell auctionator I want to buy 60 and it will find the cheapest 60 on the AH?

One of the FAQs that Zirco wrote was that this was not possible. It appears that the current API does not support this behavior.