Facebook Website Conversion Ads

I noticed Facebook has ads called "Website Conversions", where they provide a tracking pixel that you put onto your order confirmation page (or anywhere else you'd like). Then, after someone clicks your ad and converts, they have a record and you get charged. As far as I can tell though, impressions and clicks are completely free.

Here are some questions for anyone more familiar with this form of advertising. I feel like I must be missing something:

1. How do they prevent fraud or cheating?
ie: I have a website with pages A, B and C. My Facebook ad directs you to page A where you can buy my product. After purchase you are sent to page B for confirmation. However, page C is the one with the tracking pixel, which never gets shown to the customer. If Facebook doesn't have a way to detect this, then I have virtually unlimited free advertising for no cost. How do they avoid this?
(For the record I'm not interested in actually doing this, I'm just trying to understand because this sounds too good to be true)

2. Regardless of the scenario in question 1, am I even understanding this correctly? In other words, if I want to advertise my product which makes me $50 per sale, I can just pay Facebook $25 or $30 to make the sales for me? With 0* risk?

*Not counting cancellations

Thought I would ask here before I get too excited about this and blow a bunch of money.
I've tried to find answers to this online but I'm having trouble. Thanks for any input.