CRAH would be great for buyers but relatively terrible for sellers. Primarily, it would give the buyers far more control over the markets than they have now. It wouldn't automatically crash the markets, as the supply vs demand ratio would stay roughly the same, like settling in around the level seen on a mid-pop realm, though I believe prices would drop for different reasons.
The more people involved in an AH system, the more stable it will be. It is more difficult for a single player, or consortium of players, to hold control of a market. Thus much more difficult for artificial manipulation of prices to occur. This would lead to lower prices for most anything, as the market would settle into a pricing structure that more accurately reflects what buyers think an item is worth, instead of what the sellers want to get for it. Both supply and demand would be evened out across all servers, leading to more consistent prices for goods.
I really think it is only a matter of time. Blizzard seems like they are moving more and more toward a model of one single "realm" instead of a many isolated servers. Over time I expect more and more walls to come down keeping the individual realms separate. The only thing that might stop this would be Blizzard deciding the revenue from character transfers is too profitable to diminish in any way. And even that I think would only delay it for a time.