I remember hearing a little while ago that Kemp and Ethier probably can't be traded, because with MLB overseeing operations and wanting to force McCourt to sell, they aren't going to greenlight any deals for their more marketable names.
As for the general question, given their salary situation, the only reason they would shop to see what they get for Brown is if they've soured on his potential - they aren't going to trade him for someone they have to pay right away.  As for a guy like Cosart and others who are further away, I could see them doing that, but again, they probably aren't that interested in going out of their way to trade away young, cheap talent.