I never thought to look for anything interesting by the Celestial Court itself. I figured it was static, like every other quest hub in the game, more or less. I thought the two vendors standing next to the emperor were the only ones who sold anything of note.
Then I stumbled across Whizzig <Merchant of Time> in Wowhead's excellent Timeless Isle guide. I was sure I'd seen this snail-riding gentlemen before. But it never occurred to me that he was, in fact, a "rare vendor." He sells four different vanity items for your Perky Pug, such as Dread Pirate Costume. He also sells Magical Pet Biscuits and (for some reason) a glyph recipe. All of his items cost Timeless Coins. If you've got thousands of extra coins laying around, you might as well spoil your pup.
Once I made this discovery, I took a closer look around the court. There are so many NPCs and so many of them sell useless things that it's easy to overlook things you might actually care about. At this point I also found a pandaren boy named Ku-Mo <Hand-Made Kites>. He sells the Sky Lantern pet for 7500 coins and a consumable Golden Glider for 1000. The latter works just like the engineer's version, but it only works on the island -- if you're willing to risk your neck on a makeshift glider a little kid cobbled together.
You can't help Golganarr to spawn, by the way. You just have to get lucky. Fortunately he has a zone-wide announcement like the Dread Ship Vazuvius, and he takes much longer to kill than the average rare.
Filed under: Mists of Pandaria