I’m trying to get the pet “Rarity” and there doesn’t seem to be anything else to unlock the vendor than to /nod to him when he emotes.
But reviewing the comments on Wowhead makes me think this is broken.
One comment was how he whispers 3 times a minute. I’m getting the emote … like… ONCE and that’s it. I’ve waited 5-10 MINUTES … and still not a second emote.
I went and killed another boss hoping to “reset” him, came back, and while he did emote, I immediately clicked my /nod macro … and he’s still locked. People are saying you need to interact with him “within a couple seconds”, I’m doing it in less than a second.
I’ve tried /yes
I am the leader of a group. I’ve tried it solo.
I am in Mythic, but tried Hc, too.
I’ve this this, that, the other thing.
Nothing seems to work. All my various attempts seem slightly different than what I’m reading in some of the comments on Wowhead. It feels like it’s broken, but it’s hard to say since it’s a “secret” and maybe I’m just doing something wrong.
Any advice?
Fight with the RNG. The activation of the vendor is unreliable. I don’t recall finding anything much that I could do to increase my success at triggering his vendor transformation. Just keep trying, spamming the /nod at various distances (right on top of the NPC, at a slight distance, waaay out there) until either they respond or you give up.
So to check, I just sent my mage there. Mythic. I already have Rarity, but it doesn’t seem to have made a difference.
I did a /nod about 20 times while moving around after he whispered. Then he whispered again. and my next /nod opened him - I was able to click on him to see his wares. (Hmm…Istill don’t have the Rat Soul.)
Can you comment on the frequency of the emote?
It sounds like you were getting it quite often. Which is consistent with the Wowhead comments. But totally NOT what I was getting.
Once every 10+ mins isn’t conducive to “trying things out.” lol
But I’ll try again, ty. 
I got one when I approached him, but I wasn’t ready, and took a few seconds to open chat and type /nod. After that, I just used up-arrow to repeat (with Prat addon).
I imagine I took three of four seconds between /nod attempts as I moved, stopped, moved, stopped.
So his second emote would have been about 1 minute after the first. So that matches well with the Wowhead comments, and it happened today, so it is not some new bug.
Just tried again… and it worked!!
Two theories:
1> First attempts, I had Warmode On. This shouldn’t matter, but I could make a super RP case that a “secret”, sorta black market vendor wouldn’t want to expose himself to someone that’s flagged. But I can’t think of anything in the game that works that way.
2> You know certain vendors have “limited supply” items, and you need to check every 2-3 hours for them to “restock”? Maybe this guy works the same way, but instead of seeing an empty selection, he simply doesn’t unlock.
That seems dumb to me, since it’s an instance. And it’s a SECRET. lol The only way you’re going to interact with him is if you already know what’s going on so… cmon Blizzard. Just let us interact with him.
Anyway, thanks all for the help!!