I was buying a lot of used clothing for wool rugs and just threw them in the washer with regular detergent. Then I could take my time cutting them up and they were clean when I did it. I did not want bags of questionable used clothing sitting around. I was amazed just how little they changed after washing the whole suit or such. I can get more felting later if I need it but they are amazing to use as is.
I now only buy pieces that have fantastic color. Once found a hot pink wool skirt at a resale shop.