#1 is "due diligence"...don't take anything a seller posts as being the truth, as lots of sellers are dishonest on popular marketplaces, such as Flippa. Look into the stats, content, monetary claims, etc, and see if it really makes sense.
The most common claim that sellers make is that they are selling their site because they "don't have time to run it", yet in the same selling pitch they will post that it's easy to run and the new buyer will only need a few minutes of their time to maintain it. Well, it can't be both, so which is it? Either it takes a lot of time, or it takes no time....lol
Also, beware of fake screenshots. LOTS of that on Flippa too. Some of the high end sales are legit, but most of the sites under 5k are worthless. I've been scammed a couple of times and tried to learn from it, but I still find it frustrating weeding out the good from the bad.
If it seems to good to be true, then follow your gut instinct!