What is the cool-down period on reselling items on the Real Money 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.