AH: Selling fast-selling, common, and cheap items.
MW: Selling slow-selling, rare, expensive items.
This is almost exactly how AH and bazaars worked in FFXI, Aion, and practically every MMO that had both.
People will naturally flock to the AH to make their quick sales, but when they know they have an expensive, rare item that will take a while to sell, they'll put up a bazaar or shout in trade chat to pawn it off. Nobody wants to re-list an expensive item several times, when you can do it just once in a bazaar.
Even if you have no limits on the kinds of items you can sell in both systems, people will still be doing this.