It’s easy to get wrapped up in the instant and overpay for a site, but if you do that than it’ll take you longer to be moneymaking, or you may just get taken completely and never earn money from a certain site. At a minimum they should offer you screen shots showing their traffic counts, and their monetization efforts.
Lots of the larger flipping sites will have fairly stringent axioms that need to be followed by anyone who is listing a website for sale, these can help protect you but finally you are answerable for defending yourself.
Take some time to do your own research into the niche the internet site is based on. How many products are for sale in this niche? Go to Amazon.com and do a search for the precise niche items to see how popular they are. Go to the internet site yourself, look around. Most sites that sell websites will have fairly intensive coaching materials to guide you through the method, I strongly recommend that you take time to test them out first, that’s step one when attempting to find great, lucrative domains for sale.