It is perfectly fine. Just make sure you do your due diligence and try and learn as much as you can about the snake you're considering. You don't want to end up getting a good deal on someone else's problems (unless you're prepared to do some "rescue work").

Beyond that, all that's left for you is to provide the proper home that it needs to thrive. In spite of what many people think, each species of snake needs to be cared for in a specific way (temperatures, humidity, feeding, etc). This site is a good place to learn what those things are. As long as you know what these care requirements are for your new ball python ahead of time, and you are prepared to do those things, you shouldn't have any issues.

Welcome to the site!