Well you can ask this question everytime anything happens with anything. Do you sue a company that makes hot plates if you're stupid enough to press your hand on it while it's hot? No....
With such an attitude you wouldn't be able to sell even a packet of peanuts because someone may eat them and die, then their family may sue your ass for selling an alergic person peanuts.... sadly enough this could actually happen so I understand your concerns to a degree.
You miss the point a little bit....it is not illegal to sell hotplates or peanuts to anyone (well not yet anyway) but fireworks are a different matter. Most eBay sellers use PAYPAL, I know a lot of minors that have access to eBay and could easily pay for these "age restricted items".
If you are in the business of selling something that is strictly governed by law (and yes I agree that some of the laws are too restrictive) then it is fair for me to get annoyed when others blatantly flout these laws. Make it a level playing field and ALL legitimate vendors will benefit.