I've sold clothing to Plato's Closet. Basically you bring your clothes in, and they tell you to come back in about 30 mins. They tell you how much they've decided to use and how much you'll get, so you do get your money on the spot. I have to warn you, though, it's a total jip. I had some nice jeans that I decided to see if I could sell at Plato's and they only took one pair, and it was the pair that was frayed at the bottom. I also had a pair of white tennis shoes that had obviously never been worn or scuffed and they wouldn't take those, either.I walked in with a ton of clothes (none of which were in bad condition, except for the frayed jeans that they took) and walked out with $9. I just put my clothes in those charity bins now, as it makes me feel better anyway.
I believe they give you money right then and there.
You just go there, fill out a form, and give them the clothes you have. Then, they'll go through your clothes and give anything that they can't sell back to you, and then they give you money for the clothes they can sell.
Here is their website, which tells you how it works - http://funadvice.com/r/14v0tnjt8g3