I sorta disagree with you. I have met some very very experienced teen keepers. The OP said she had had experience with non feeding snakes. She (what sounded like to me) implied she had a high level of experience with balls. At what point do you trust a person to know their level of skill? IMHO it was her parents duty to know what she is getting into. Yes the seller SHOULD have warned her. But, again IMHO, to not sell something to a person just because they are a teen is wrong. How do you know that the next person, an adult, saying the same thing is any better? When do you trust their word about their level of experience? Is a teen that has had 4 years experience any worse than an adult (and at what age do you consider a person a responsible adult) that has had the same amount of experience? How long do you have to have snakes to be experienced?
If it where me I would never had had this animal up for sale to the general public, for the simple reason I don't trust people (no matter how old they are). I would have either sold or given it away to someone I absoultly KNEW could help a snake that was a non feeder.
At one time or another most of us overestimate what we think we know. Who's fault is it? The OP learned 2 very important lessons. First thats she was not nearly experienced as she thought she was. Second that when you over estimate yourself bad things happen. The saddest part is a poor snake had to die for her to learn this.
This could have just as easily happened to an "adult" as to a teen.