http://images.craigslist.org/3ne3k33...3c3b3e14f3.jpg
http://images.craigslist.org/3n23o63...1635161d0a.jpg
The ad says it is a rare Lampropeltis calligaster rhombomaculata. "What makes this kingsnake rare though is that it's primary color is green as opposed to grey/brown."
I googled that scientific name, and came up with mole king snake, but the pics don't look like that snake.
My best guess was Thayeri, but I don't know kings very well.
They only want $40 for the snake & tank, which appears to be a 20g long with screen top, so I'm tempted, but would want to be sure on the species before I acquired it.