I heard a clerk talking the last time I was in Goodwill and they were saying most of teh good stuff gets culled and sold online
http://www.shopgoodwill.com/
and then they mark the rest and only mark it down to empty the store.
My Salvation Army has half price days so I tend to shop on those days, or go to the smaller local thrift stores when I want a cheap bargain.
We do have a local chain called Value Village which sells used clothes but they are very clean and organized and fix things up a bit, so even though they are priceier I don;t mind because everything is clean and in good repair.