Looking forward to buy a Site from Flippa or FE International

Hi everyone, I am looking forward to buy a site either from Flippa or FE International. Some sites costs over 1500 USD+, hence I wanna be completely sure before I invest such a good amount.

What I wanna know is :
1 – Those sites that say that the buyer will be able to make 1000 USD+ approx per month, is it true? Is it really possible?

2 – Why will the previous owner sell such a great site that was already making him 1000 USD approx per month? Also, I have seen some sites generating around…

Looking forward to buy a Site from Flippa or FE International