If I was Seattle's GM

...I would select:

Defense:
From Colorado: Facundo Erpen. He's a proven commodity with a lot of experience. Erpen was my last defensive selection, and you could take Jim Curtin instead.
From New England: Gabriel Badilla. His previous club was Saprissa, and would start for most MLS teams.
From Dallas: Duilio Davino. Sure, he's 32, but he's got Mexican national team experience, and at times he can look like one of the best defenders in the league.
From New York: Chris Leitch. Must have done something right to get his team to the MLS Cup. And I'm unsure about taking Jeff Parke with the suspension.

Midfield:
From Chicago: Marco Pappa. This young dynamic Guatemalan has only a year left on his loan, but could be an impact player.
From Columbus: Eddie Gaven. I'm just not sure how this guy got left exposed. Did they forget? Or is he maybe trying to go to Europe?
From Real Salt Lake: Nathan Sturgis. Young versatile Olympian can play defense or midfield.
From Toronta: Rohan Ricketts. This former EPL player knows how to score and knows how to win. A Ricketts/Lljunberg combination could be dangerous.

Forwards:
From Houston: Nate Jaqua. Although I would rather take Ianni or Barrett from Houston, the word on the street is that Jaqua's move to Seattle has been agreed to long ago.
From DC: Francis Doe. I hate to do this because I don't want to lose him, but Doe would do well next to Jaqua with some quality midfield service.

Disclaimer: I paid absolutely no attention to salary or international status in making these selections.