best used record store s for "BLUES"?

http://www.gemm.com/

bop street in Seattle. Dave, the owner, used to host a blues radio show

Don't know if it's the "best", but Princeton Record Exchange in Princeton, NJ, has a wide selection of used LP's. Here's the link to their web site: http://www.prex.com/

If you are ever in Princeton, plan to spend a chunk of time browsing their inventory.

Discogs? online