This. A large portion of common law revolves around intent - I think the technical term is "mens rea" (mentioned by another poster). If a site sold knives as

This. A large portion of common law revolves around intent - I think the technical term is "mens rea" (mentioned by another poster). If a site sold knives as "neighbor killers", with the comment "use this and you can definitely kill your neighbor, $19.95", then all the same considerations would come into play. And knives aren't illegal, at least to cook with.