Only one thing ?
Hmm.. Shell isn't actually one thing , but more one company with a lot of things.
Company wise :
I think i'd buy Microsoft , pull 1 billion dollars out of it and let the entire company die.
Item wise :
If i could buy all things of an item , i'd buy all food in the world (why yes , we can) and distribute it properly.
Or all weapons , and destroy them.
Or all water , and distribute it properly.
Or all internet domains and rule the internet world.