According to this post Auction House FAQ Updated - Diablo III AH is suposed to end around 3 am 18.3. and since that moment buyout and bid option will be disabled and all items shoud be delivered to the highest bidder.
They stated it might happen little earlier or little later around 3.am so i would supose it would be safe to make some around 2:30 probably or just try your luck.
There are many items in AH with low starting bid and if you throw bids on those items you will get them the moment AH turns offline. With this you could get some good gear for RoS opening for realy low prices.