I live in Seattle and the area *is* thrift store rich. I think here are several factors feeding the market: population (not too big, not too small), a wide range of outdoor sports, and a lot of manufacturers in the area. I can't tell you if some local tradition of donating to thrifts is a factor, but I wonder. Of course the economy, average income, and age must be factors too.
I have made a lot of Ebay sales to rural areas all over the US for no other reason than the lack of access for the rural residents. With all the high tech in the area, as well as being a regional hub, there are all kinds of what I call "corporate dandruff." At one point I was buying HP DeskJet 600 series power supplies/AC adapters for $0.99 and selling them for $10 on Ebay, etc, etc.
To be fair, this is a hobby for me. I might make 10 thrift store stops a week. Much of it is being in the right place at the right time, and time spent equals deals found. Several of the water bottle rigs were put out at Goodwill minutes before. I have developed a practiced eye too. I can go down a row of used clothing at a near walking pace and pick off the outdoor stuff. After a while you get used to the fabrics, colors, and details. It's weird but your go through this stage of dealing with all the abundance-- you want to buy every deal you see and after while, you find you have all this stuff and need to have a garage sale, Ebay it--- or end up donating it. My wife works a couple weeks each summer as a camp nurse and we send along a big bag of fleece and rain gear for the city kids who show up with inappropriate clothes for camping and kayaking.
At this point I don't even look at clothing unless it's better brands like Patagonia, North Face, Prana, Marmot, Ex Officio, etc. I don't find a lot of true ultralight gear-- I've found a few GoLite items. Most of what I get is basic synthetic outdoor clothing.
The garage sale season is about to start up. Can't wait!
I do take requests if you are looking for specifics :)