Firstly, if you have doubts on whether or not you should purchase the snake, its probably a good idea that you don't.

Secondly, the snake could have been a very small hatchling. I would ask for his hatch date and hatch weight. If you take into consideration that hatchlings can be as large as 70-80g at hatching, then yes, at 5 months they could easily be 200g. A hatchling that hatches out at 30-40g might take a while longer to gain the weight.