Any system can be simple to implement as long as you narrow down the scope or features you would implement.
Inventory System is a bad idea unless you specify the actual scope.
I can tell you to make a seemingly simple system like print out Hello World system. But if I tell you the scope is to print out Hello World in any possible language available and print out could be text, screen, paper, API --- suddenly your hello world system is now one complicated beast.
Now think of the most complicated system and if the acceptance criteria is to just print out the title as an html string, well that's easy. It's stupid but easy.