A large percentage of ball pythons are very picky eaters, as this one may be which would account for it's small size. I always prefer to buy them as hatchlings and raise them myself. You should be able to get a hatchling ball python for about $25 or so and still be able to buy the rest of your supplies and end up right around the $100 you'd spend on that one. I find ball pythons boring, but if it's your first one I guess it'd be alright.
even after your snake takes a live mouse you should watch it to make sure the mouse/rat is not bitting it while the snake restricts it. yea the reason there boring is that they are nocturnal. my snakes all over the tank at night.