They said that, "Marketplace will be similar to Icecrown model."
We cant say for sure. They will probably wait some time before they even sell heroic items/shadowmourne.
How long does it take for one guild to farm shadowmourne?
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BGs are cross server so I don't see the point of someone getting another SM. Most people who can buy SM already have it.
http://forum.warmane.com/showthread.php?t=415559 They addressed the crossrealm in the original post.