I bought cm boost in MOP because I cba finding 4 competent people who are actually interested in cms and aren't elitist assholes asking full gold achi beforehand. And I like the phoenixes. The yeti is ugly though, so I possibly won't bother nowadays.
People buy arena carries all the time because you need conquest gear to not be steamrolled in pvp, so they pay someone geared to carry them to the cap especially when it's undergeared alt with huge cap to fill.
Now raid loot runs I have no clue why people buy, the gear with lose value, and if you buy carries it means you don't raid, so you get zero benefit form that gear. Unless they're buying stuff like Blackhand mount, which will stay with you.