I'm not Seattle really, but your question needs a reply.
Maybe the answer lies in a better definition of location and price range.
Recycled Cycles - Recycled Cycles. new and used
Gregg's Cycles Hours and Locations. - Gregg's Cycles (greggscycles.com)
3 locations in Puget Sound
REI in Seattle offers Pure SS
Specialty Bikes | REI Co-op
There are many local neighborhood bike shops, both New and New+Used type shops.
Obviously, call ahead for stock and sizing.
If you are looking for an affordable rider, check Seattle.CraigsList, before arriving. Lots of bikes posted every day, many from neighborhood bike shops = fully serviced and ready to go - usually with minimal/no warranty or guarantee. But you can get a good check ride with appropriate payment up front.
And the other online local shopping sites have bikes.
I got lots of bikes - no fixie or single speed. Lots of hills in the city and region.