Depends, some cool T-Mogs can be sold for about ~60k gold on the AH. (rare robes for example, after Cataclysm no longer obtainable with unique model)
So it's going like that:
-it looks stupid/not unique model - vendoring it,
-it looks cool/nice model/has a big value - checking AH/WoWHead/Other sources -AH it.
I sold a stupid green staff for 1,5k.
Also found 3x BiS trinkets in 6.0.
(I'm only playing on one character, and that's my main)