Not consignment/used, but I picked up a tip from a friend who *buys ahead* with sales at Old Navy. I found the new clothes on the seasonal sale and clearance (rather than the specials on this seasons stuff) are the same price or less in general than clothes in the consignment shops. So last fall I stocked up on t-shirts and shorts for my son for this summer, they were around $1.99 - $3.99 each.
There's a resale on SE Division around 30th, I think, that I've been wanting to check, it always looks packed with stuff. I've never had any luck at Goodwill, for how many clothes I know get donated, there wasn't much on the racks.
I also wonder what happens to all the gently worn stuff. We tried to consign stuff that was hardly touched, and they rejected bags and took like 1 shirt. It was all current and good quality. I don't know why the consignment stores don't just take it and sell them for $2, instead of trying to just stick with things they can sell for $8. It makes me not use the consignment stores for either side (especially because of the Old Navy thing I'm doing now).
Anyway good luck, I'm going to keep checking on the advice you get!