When it comes to profiling sellers on the trade market, it helps for the rebuilding team in question to have a star nearing free agency. That kind of scenario typically makes the process of finding a trade easier; the bad team has something that all the good teams want, and with the star possibly on his way out the door, the club has some incentive to get something in return while they can.
But when a selling team lacks a stud at the top of their roster to shop, well, it can be harder to find a meaningful deal.