I tend to use the message board to post that I'm looking to move someone or when I'm looking for a certain position to fill... Kinda has the same effect.
But some people probably don't want to have to type anything and are a little hesitant to post.... I like the idea
Have an extra checkbox next to players names that can be checked. And when others look at your team those that are checked will be marked as players that you want to shop.