Um. No. It's obviously a scam. They're selling three pets yet you say it's because they have a very old account? So, what? They have three accounts that they will transfer to you then? For those prices, I can't imagine they'd think to get an all for one price (at least, if they mentioned the different prices, then it seems they haven't.)
So what are the chances of them having three vanilla wow accounts with two of the most rare pets in the game and are then selling them for way under half the going price? Hm... Last time I checked, Murky went for 2k. For $700, I'm sure they'll think people will think it's high enough to be 'legit'. I mean, if they tried to sell it for $30, it would be obvious.