Obviously we're stuck with Pujols and Hamilton. You don't trade Weaver, Trumbo, or Trout. Those are the only absolute untouchables to me. I know Trumbo might seem like an enticing piece but he will still be in his prime if the team is better in a couple of years, you keep him around for the future. He's only going to get better, if we traded him he'd probably make us pay like Napoli. He's also a hometown guy who will likely take a hometown discount in the future. Weaver will turn it around (hopefully, see I can be optimistic) and is the anchor of the staff, and Trout is Trout.
But I'd definitely trade Kendrick and Vargas. Those guys will net you back a lot and wouldn't cripple the team's chances of contending in the next few years like a Trumbo trade could. It would be smart to sell high on Kendrick, he's been a solid player for us for the last 7 and a half years but who knows if he'll hit .300 again next year, trade him while his value his high. Vargas has been solid and this is his walk year, I could see a contender giving us a solid prospect or two.
Bourjos I'd consider as well. I love Petey but he's having a good year right now, and if we sell high we'd get a solid return. You only trade Bourjos though if you have a real fire sale. If they're just looking to stock up the farm system a bit you stick to Vargas, Kendrick, and a couple other guys. Frieri is another guy who like Bourjos would be tough to trade but you'd get a good haul. Anybody else in the bullpen I'd trade with no hesitation, we should really be starting Williams right now so we could get something good back for him. Him being in the pen hurts his trade value. We definitely have a lot of good pieces to offer though. Unfortunately these guys just can't seem to win consistently together (in large part due to the guys we can't trade unfortunately).
Another idea is CJ Wilson. He's looked better lately and if somebody would take on the contract and give us a couple of prospects, you definitely deal him if he'd accept the city.
Also if Hamilton has a hot 2nd half, try hard to trade him in the offseason. That's a pipe dream though, lol.
Aybar is having a down year so I'd hold on to him, you don't sell low on a guy like Aybar.