Diablo – buy gold from the cash auction house

As far as I can tell, there isn't one. I purchased an item at the auction house using Buyout so that I would know immediately when the auction ended. After a few seconds pause my buyout was accepted and the item was available in my Completed tab. I clicked to move it to my stash, and then went to the Sell tab and was able to immediately put the item back up for auction.

All Diablo 3 Auction Houses (both Real Money and gold) are regional, one of the same three regions there are for servers:

The Americas, Asia, and Europe.

Additionally, Hardcore characters are entirely separate from "softcore" characters - this means no trading and a separate shared stash, but it also means a separate pair of auction houses to boot.