I'm sorry if I'm wrong but recently I found info about scaling old items due challenging system in MoP which is not clear too much but also I've spoke to people who seems to be on beta and they do confirm it. So only if they will scale weapons / gear like old legendary
1. Sulfuron Hammer - Item - World of Warcraft
2. The Sacred and the Corrupt - Quest - World of Warcraft
3. Enchanted Elementium Bar - Item - World of Warcraft
those are items which have BoE requirements (mats) and if they will scale those to challenge modes then due their BiS (like thundefury) "options" those mats could get through AH's price roof.
And if they won't there is still something involved with legendaries.
They (FoS achievs) are now account wide then why don't sell boosts included them like run in instances with twinks, selling hammers.
Also I think I wont discovery new America but if you have 10 x 85 you could get almost guarantee* boss drops from anywhere. Mounts/meta achievs mostly are also account wide - 10 x karazhan / lk etc with prepared offers to buy mounts (just to obtain them and forget about them).
If anyone could confirm/deny legendaries scaling I'd be grateful.