well, I don't know about this from first-hand experience as I've never bought or sold a website, however as someone who has good experience with many aspects of web development and growing experience with internet marketing / black hat methods I can say this....
basically, a money-making website is often not something that can be left "static" and just sit there raking in cash for an infinite amount of time. Many internet projects (websites), ESPECIALLY those built with black hat methods, have sort of a bell shaped curve of their life timeline in that they peak and then die off, either gradually or quickly. Unless you maintain a spot as a top website for a niche, the site is going to hit a point where its at its best with google then it may instantly die off at some point, if black hat methods were used and the site is sandboxed, or may gradually be replaced by other websites in the searches.
It certainly is possible to have a site made with just initial work and then stay up in the search results for a long time but often the people making these are looking for a quick buck so they use whatever methods possible to drive it up to the top of the results quickly and don't give a shit how it ends up a few months later.
There is a lot involved in putting out web content, by this I mean you need to be familiar with the process, have a team of writers, re-writers, article spinners, etc or you are not going to be able to add a good volume of content regularly. Then the only other methods to get new traffic are not passive, rather they are things like social media marketing. This can be automated and made passive in some sense, but this isn't going to come with a website.
Overall, my point in this is that its better to learn the ins and outs before even considering buying something because you may find after buying it you don't know what to do to maintain the traffic or may find that it suspiciously takes a nosedive.
