I dont know if you can consider it a legit way to buy gold, because I wonder how long it will work and how much you will get out of it.
I mean, my theory is:
a) The people who love the pet or are really into pet-collecting and pay their account with a credit card might just get it that way, simply buying it with their account at the Blizz store. I am rather certain some people will prefer this over buying it for gold ingame.
b) I doubt you will get very much out of it. I am thinking of something who, I dunno, buys 10 of them and puts them on the AH. How much could those go for, how much are people willing to pay? 5K? 10? 20? (Keep in mind what I said above... a lot of people will prefer to buy from the Blizz store themselves)
Eventually, I believe the market will run dry. So even if you get a decent amount out of it (ofc, what 10 bucks is worth to you in ingame-currency is up to you), there will be a limited amount of players buying it...eventually those that want one and cant or dont want to buy from the Blizz store themselves will have theirs, and I dont know if the amount of new players coming in will guarantee that you'll always have enough people wanting to buy one.

Ofc, I might be totally wrong. Maybe there are bunches of people who have too much gold and would like another gold-sink, maybe enough for those people who will buy several to put them up.

What I DO like is that you can gift it to friends even if you don't have their email-address. Some people dont wanna give that out, and so if you know a guildie has his birthday, you can now giftwrap (maybe not) that pet and send it ingame or hand it over =)