The Mets won't be the only sellers at the trade deadline, if they will even be sellers. There are other teams who would do well to unload some stars for prospects this July.
What makes a star movable? A hefty contract can certainly make a player less desirable to his own team, but it makes him difficult to move (though that didn't stop the Blue Jays from unloading the worst contract in baseball in Vernon Wells).
If the team is struggling and doesn't have a deep minor league system, they could be motivated to replenish the camp by trading one current star, for three guys who could become stars.