Personally I must say I kind of like this idea, not sure if it would be that easy to implement tough.
Could be something like a simple WTB AH tab, and I supposed a timer would need to be associated with the purchase order as well.

But to be honest I think we would see some sort of reversed undercutting between buyers.
Let’s imagine for example:

Player A: Wants to buy 10 Fight Club at 1337 gold each.
Player B: Sees Player A’s offer and decides he wants to buy 10 Fight Club at 1338 each.
Player C: See’s all the available options and sells to the “highest bidder” = Player B.

I would still have a few concerns though, this would probably bring excessive complexity to the AH, because for example, if you are a seller, you are probably going to end up doing some extra work, not only you will have to check the current AH prices from other sellers (competitors), but you would also check the buyers offers against one another and against the current sellers prices in order to find the option that would benefit you the most.

Buyers would see a similar competition to sellers when they face constant undercutting. This would probably create a certain frustration because when you are a buyer, chances are you need those items more urgently than when you are a seller, since sellers only get gold.
So the sense of urgency from buyers would probably work against the success of this implementation.

Still I think this is a good idea and I’m probably exaggerating a bit when I speculate about the possible consequences. We just need to make sure that ideas are sound before we spend resources on them, we need to be sure that not only they would work, but that enough players would actually benefit and experience better gameplay because of it.
I’ll share the suggestion with the devs, but I suspect this isn’t something new to them, we’ll see.