Asking a ten year old to sign a poison book sounds like very good education: it it clearly stuck with you, and may have helped you become a responsible person who can handle dangerous substances. May even have saved your life.
Perhaps we could do something like that with some 'adults' who buy things like fireworks or other dangerous products (including myself and sharp objects like end mills – blood does SO spoil the surface finish).
Perhaps what needs pointing out in the above exchange is that a fair amount of this dangerous stuff being sold on eBay IS illegal to sell in this way and should be reported. Although it might be worth considering that it may not be what it says it is: no one is going to report to trading standards that their illegally purchased acid is not full strength.
Anything dubious through the post really should be reported to Royal Mail out of respect for their workers, who would be in the front line if anything went wrong. They can can enforce pretty extreme sanctions on businesses and individuals, I understand.
Matthew