Over here on Paypal are instructions for creating one of those little "Buy Now!" buttons. The short version is, it involves you typing in information like how much you want the item to cost, and when you're done, Paypal gives you a bunch of code to copy. You copy the code, put it on your website, and that makes one of those little "Buy Now" buttons show up on your website, linked to your Paypal account.
If someone buys a thing from you using that button, then Paypal will take the money from them and transfer it into your Paypal account. It's possible to link your Paypal account to a bank account if you want to put the money in the bank.
That's the very basic version of how selling stuff through paypal works! If you're ordering something physical that you would ship out to a buyer, it gets a little more complicated since you have to worry about shipping stuff.