I would be guessing pullet, I have only had araucana crosses and I know they are hard to tell, one of mine has only started to change in the last week or so at around 12 to 13 weeks, but as mentioned mine are x's, so I am sure someone with more knowledge will chime in. Good luck.
The bird is neither blue nor a pure bred Orpington:( - far too thin and there is some rogue barring on his chest. He sure is a rooster, though. At least the seller mentions that he/she is not sure about him being pure bred.