This is rubbish. Under 18's can sell domains, and at any price they wish, as long as it belongs to them. You could not sue someone for damages, just because they bought a domain from your son/daughter at less than its worth!
To be honest, when you get scammed, it's likely to be someone in a foreign country, which you can do nothing about anyway as you can't hold someone resposible if you don't know who they are. So being under 18 really doesn't make much difference on the internet. Trust is the main factor, if you can't trust someone, don't do business with them. I believe, that people who don't do business with under 18's will lose out in the long term. Same as a shopkeeper saying "I won't hire him, he is under 18 and I can't hold him accountable".