A few years ago I wanted to build an affiliate website that could be fully automated to see how much money it could make me. Now, that website makes me around a thousand dollars of profit per month.
Building the website is the easiest step if you want to make passive money online. Getting traffic and convincing people to click the buy button on your site is much harder.
To earn money on the website, I just forward the users to a real webshop. The real webshop then notice that the users are coming from my website, and then I get a little cut from the sale.