Well I just got my first ball python and its gorgeous. I cant pretend to be an expert but I can offer you some advice I picked up.

The snake should be in a clean, properly kitted of home. The ex-owner should show knowlege of the animal to show that they have cared enough to find out.

As far as the snake is concerned you should be sure it is feeding well. Ask to see it feed if you wish. The snake should be perfect to look at, i.e. no evidence of injury, smooth body etc. There are probably more qualified people to tell you what to look for here.

Make sure you handle the snake to see how you react to each other. These snakes ARE as placid as they are said to be, I could belive this myself until I held it the other day.

Apart from that, it seem that in the few days I've had my snake, it was a great desision to buy it. Any other question I will help with if I can, as a fellow new comer to snake owning