From the Blizzard Auction House FAQ:
If I no longer need an item I bought in the auction house, can I relist it in the auction house?
Yes. Once you've purchased an item you can do anything with it that you could if you had acquired it through your own adventures, whether that be using it yourself, or, after a cool-down period, trading it to another character or relisting it on either the gold-based or currency-based auction house.
EDIT: I know for the Gold auction house, there is no cooldown, but I was wondering if there was one for the RMAH. I don't want to put down cash to test this out, but I would also like to know how long I have to wait before reselling an item or trading it to a friend.
How long is the cool-down period for reselling/trading items purchased from the RMAH, if any at all?