how big does O. Mercatoris get. there is one for sale at my lfs and i am thinking about purchasing it. Is it an active octo or will it spend most of the day hiding?

check out this craigslist.com posting. can anyone tell me what kind of octo this guy has. i really would like to pick it up if its bimac or vulgaris. any ideas
http://sfbay.craigslist.org/eby/for/233874885.html

My mercatoris is 100% nocturnal. Their mantle only reaches 2cm, less than an inch.