Having traded lots of WoW pets, and sold lots of stuff on ebay...
Selling a physical item w/ an unused code (or a used code for that matter) is allowed (IE. A TCG card, or an unused Blizzcon Murky card). You should however, make damn sure you specify if the code is used or not, or you could risk a dispute from the buyer.
That's true of all games/digital content, not just WoW. If it's not a physical item that can be mailed, ebay doesn't allow it.
As such, selling Hatespark (or other pets that are in no way physical real world objects) on ebay violates both WoW and ebay's terms.
THAT said, 100s of people list digital content anyway, and there's far too much of it for ebay to actively police, so as long as people keep their listings relatively low-key, and no one reports them, people still get away with it all the time. Usually they get listed and sold in a short window, before ebay ever even takes notice.
Some people even attempt to sidestep it with listings along the line of, "You are buying this napkin, as an added bonus, if you buy the napkin, I will also e-mail you a promotional DLC code" ... making the napkin the actual subject of the listing, and the code incidental. (I'm pretty sure this might also not be allowed, but people still do it. No one ever actually mails the napkin
Many listings, for say a piece of Gamestop DLC (as an example) also SAY they're selling a physical card, but it's more or less understood by those shopping them that no card will ever actually be mailed, and the code will just be e-mailed. The listings SAY they'll mail a card, just to adhere to ebay's listing rules (as a buyer, you could actually report the seller for not sending a physical card, but... that would be dirty pool, and most people don't care that they never get the card. I'm sure people have probably gotten into squabbles over it though. I don't think it's an issue with WoW TCG cards, because those are a collectible and pretty much always do actually get mailed. But DLC gamestop cards/inserts, once the code is used, are worthless, obviously, so most people don't bother mailing them or demanding they be mailed.)
SO... ALL that said, you can legally sell WoW Collector's Edition codes on ebay. You can legally sell physical blizzcon card codes on ebay. You can legally sell TCG cards with codes on ebay. You can even legally sell the Gryphon Hatchling and Wind Rider Cub Code Card on ebay (w/ or w/o doll)... And they can all either be used or unused. (but again, make sure you specify)
I actually had trouble selling some plastic WoW cups once, because ebay was detecting WoW-Related words in my listing title (Warcraft, Lich King, Deathwing, etc) and thought I was trying to sell digital content. I had to creatively word my title around them (War-craft) just to get the damn cups listed. So even their automated, non-moderated systems try to prevent this, but people still get around it easily enough.