In one year leagues, Kazmir should be fine all season. Oakland is a great team with a pitcher friendly ballpark, plenty to like there.
Rendon has slowed a bit but he is putting up numbers that are very similar to his pre-season projections. If you are playing him at 2B, that makes for a solid guy, if you are using him at 3B, you likely want more power from that position.
Dee Gordon is a sell high in the sense that he probably won't bat over .300 the rest of the season. That said, Alex Guerrero just got a chunk of his ear ripped off by his former teammate and that means Gordon will be in the majors all season and has a legit shot at 80 steals.
Melky has looked good this season, but getting Zimmerman is a solid sell high, despite my dislike for Zimmerman in the second half. That's a good trade to make.
With Pollock and Crawford in free agency, thats not too far off from Melky's production. I assume Zimmerman would be a pretty big upgrade on the person you are dropping. I would still make that trade.
Also, if those guys are available, perhaps you could stash Gregory Polanco now before he gets called up in June.
Big fan of the Zimmermann for Melky trade as well - Also a big Pollock fan - the power seems legit based on how far he's hit the ball so far, and could produce double digit steals. Just won't have as many hits as Melky.