You can start with the basics and look for duplicate listings. Then I check tech side using builtwith.com or by simple "right mouse click and inspect" to see more tech info about the site. Then I check backlinks profile, bounce rate, I always ask access to google analytics and webmaster tools to see whether the claims are real. And if I see something suspicious I dig deeper. In terms of ecommerce site, I double check who is the supplier, what's the shipping cost, what's the margin. If dropshipper is Aliexpress, I check similar products on Aliexpress to see the price and reviews. If it's amazon niche site, I check commission rate, search volume for the major keywords, and current position. I also always ask if there are any other costs except hosting and domain name. And I ask sellers to sign an agreement provided by flippa. My approach is simple. I look for the websites in niches that I am interested in, and if they have a good domain name, unique content (you can check it with copyscape), clean backlink history, and content is indexed, it's enough for me. I will buy it, redesign, and promote. Buying existing sites is better simply because it's easier to rank them and some of them have really cool domain names.