For sale because they are earning the current owners too much money, no doubt.

Or their business model is to build up and then flip/exit.

Clearly. Whether or not they are buying traffic is a different story. I wonder how often the traffic disappears immediately after sale? Awful lot of those listings have recent traffic inflations bumping up average revenue...

It's possible, though generally Google ranking is more important than traffic. (Many sites use PBNs to impact ranking, but generally this is disclosed)