You'd think there would be more in a bigger city like Seattle, but most of them have closed. if you are just in town for a quick visit, the shops are worth a try. if you are moving here or will make frequent trips, then you might try to catch one of the shows held by WSSCA. That might be your best bet.
http://www.wsscaseattle.com
I used to travel a lot within the area for work and knew where all the shops were located, but as I said, many have since closed their doors.
Card Exchange (North seattle) and DJ's Sportscards (Renton) are the two I still visit periodically, but that is mainly for supplies (Card Exchange) and oddball, cheap stuff (DJs).
I heard that Brown's Cooperstown (a shop that was in Freemont) sold and may be open under new ownership in the Queen Anne area, but have never been.
Too bad U Trading Cards closed their doors. They were centrally located and probably benefited greatly from being close to the UW campus, for a steady flow of new customers and probably for a steady flow of new college kids selling collections for beer and tuition money!