Buyers or sellers?
If you frame it as "buy" vs. "sell," then it's a tricky question indeed, because on the one hand, the Mariners are only five games out of first in the AL West. On the other, there are two teams in front of them, they're about to lose two starting pitchers (Erik Bedard and Jarrod Washburn) to free agency, and they lost 101 games last year.
I might look for new GM Jack Zdurencik to do something that doesn't fall clearly on either side, and I'm not sure what that will be. But Zdurencik is too smart for obvious binaries. Take the recent trade of Yuniesky Betancourt to the Royals for pitching prospects Dan Cortes and Derrick Saito. Sure, that looks like selling, but when you're just getting rid of a terrible player and a terrible contract, you're probably not making the team worse.
I don't think the Mariners will give up any good prospects to make a run at a flashy veteran, so they won't be buying in the traditional sense, but they might find more creative ways to improve. One possibility would be to flip Bedard or Washburn for a player who won't be a free agent at the end of the season but can still provide immediate help.
The Mariners' catchers have been among the worst in the majors and, like a number of contending teams, they could use a bit more pitching depth, both in the bullpen and in the rotation. A real answer at shortstop--Ronny Cedeno might not be enough--could help, too.
Infield prospects--shortstop Carlos Triunfel is a high-risk prospect who could turn out to be very good, and there are a fair number of interesting third basemen in the minors, but there aren't enough middle infielders or first basemen. They could also use prospects of all stripes, particularly in the higher levels of the system. Zdurencik's predecessor Bill Bavasi didn't do the M's any favors by shipping off a number of promising youngsters for Bedard and by pursuing a bizarre drafting strategy that included drafting relievers in the first round twice in a row (with Phillippe Aumont in 2007 and Joshua Fields in 2008, depending on how you look at it; the M's under Bavasi used Aumont as a starter, but then again you might count 2006 first-rounder Brandon Morrow as a reliever too, since Bavasi used him as one).
Big leaguers on the market?
Erik Bedard and Jarrod Washburn, who are both pitching well and will both be free agents after the season, might be available. With the recent acquisition of Ryan Langerhans, outfielder Wladimir Balentien could go. And while David Aardsma's name isn't coming up with any regularity, Eric Karabell makes a compelling case that Aardsma might be the the sort of player the M's would consider trading--he's racking up saves, which are often overvalued.
Minor league strength
Their pitching depth could use some work, but Michael Saunders, Ezequiel Carrera, and Tyson Gillies are a very good corps of young outfielders. Greg Halman, who has hit a ton of homers this year and done little else, would fit into the middle of that list, too, if he were hitting better than .212. Second-overall draft pick Dustin Ackley will top the list if he signs.
Take on short-term money to win?
Dumping Betancourt should allow the Mariners to add salary at the deadline if they need to.
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