Where about in Seattle are you going to be exactly? If you can make it just outside the Seattle city limits, there is Kenmore Camera.
www.kenmorecamera.com
They are usually pricey for used gear, but always in top notch shape.
Then there is Glazers.
Welcome To Glazer's Camera | Glazer's Camera
I usually don't shop there, so I can't really recall what they may or may not have.
Then we have lots and lots of thrift stores all around the city, too. If you're persistent (and have the time), I'm sure you'll find something nice.