Buying facebook user data from third party apps? Illegal?

Hey guys. I just started learning rails and about ready to create my own web apps. So i need a facebook user data for my apps. From my understanding, i can do this in a few ways;

EIther built a facebook scraper, or buying data from third party apps.

I know there is a lot of free apps collecting user personal data and sell it to third party. But my question is if i would like to buy some data, where do i get them? Where do i look for apps owner who sell this data? And Is buy/selling data is illegal according to facebook tos?

Thanks in advance for your answer.

How exactly can that be Illegal?
How'd you acquire it?
And
Why'd they sell it to you? I thought that's super secret data meant for super secret government agencies!