The point of banned goods is not that they are banned because of the hazards for the people alone who buy them but for everyone else also. Sulphuric acid for example is easily usable as a weapon especially in concentrated form. (It grows very hot if it touches water. And it is very acidic. So, by using a simple acid proof squirt gun one can do serious damage.)

And, that's not really all: Suppose I could go into such a shop, proof that I'm sufficiently intelligent to handle dangerous stuff without being a danger for myself and buy a) a PCR machine b) a flu virus genome sequence c) a HIV genome sequence d) some assorted chemicals e) some literature about virology f) lungs tissue cell cultures g) some pigs/monkeys to test it

and wipe out Japans population just for the sake of it? (Contagious like flu and deadly as AIDS. It would take some months or even years to clone it but it would be manageable.)

Well, Japan wouldn't actually a sensible target for that. To much risk of travelers spreading the virus worldwide. Choose your own isolated country to test.

I sleep better in my bed each night because I know it is not that easy to get really dangerous stuff in Shops.