As far as big companies go I have a sneaking suspicion that it's kind of a "back door" thing when they sell a big ship. I have a hard time believing that a company like Maersk would throw up an add on a website and wait and hope to see what they pull in. If I had to guess I would say they probably use multiple sources of information to find out who might be looking for one and quietly put out feelers to see what they can come up with.
As for everyone else there is a website called Maritime Sales that always has a lot of small and large commercial ships for sale all over the world, just don't expect to find too many E-Class container ships or 1000-foot tankers. If you're looking to pick up something for the coastal trade in the Caribbean or maybe an old tug you can beat up for a few years then throw away, Maritime Sales is the place. That's not to say they don't have some nice stuff too, they do, but much of what they have is medium-sized, foreign-built, older stuff.