Is it a sin to sell an electronic cigarette it to a minor?
Please cite sources.
Since it is a felony to sell tobacco products or electronic cigarettes to minors in most civilized countries, it would be forbidden to do this based on the principle of "dina d'malchuta dina". You could end up in jail for doing something like this, I hope you don't plan on doing this.
They are bad for your health and a minor may not fully understand this. This is why there are laws against selling certain items to minors. That being the case I would think this could fit under the sun of "placing a stumbling block in front of the blind".