In the B&S and consumer forums, it's the sort of spam that might actually be considered as legal. It's the easiest way to bring your thread back up to the first page of the forum so that more people get a chance to view it.
In most cases, it's not possible to sell an item immediately after putting it on sale, and a considerable amount of bumping of the thread is necessary to gain as many viewers as possible. And when that happens, chances are you'll find a buyer.