AgathaC, in answer to your question, not necessarily. Sorry if that answer is vague but I will try to elaborate - without mentioning names.
My company carries out contractual work for three charity businesses. I know these charities are reputable (they are big names and international). The staff in the shops are volunteers, however they have a manager who is on salary. The staff in the shops have the added bonus of getting first pick of what comes out of the "bags". So this would be a perk for the volunteers.
Also some shops get "shop returns" from some of the chain stores with labels still attached. I would presume from this that would add to the credability of the charity shops.
On the otherhand, as I have stated before, I am aware of (lets say more than one shop), whereby the "owner" gets the bags of clothes as delivered, opens the top of the bags and empties them direct to the "green bags" for disposal. I was in one such shop on Thursday and there were approx 30-40 bags for collection. While I was there (for about an hour) several people showed up with bags of household items and clothes. The shop was very busy but the selling price on most of the stuff was about 2 euro. There were a few prams for a tenner each. Anecdotaly, the demographics of the people buying were very interesting - thats for another forum.
It would appear to me that the money is in the recycling of clothes - not the resale in the shop.
I asked the owner how much he has paid to the charity that is named over the door.
He looked at me, smiled, and went back to refilling the bags.
Draw your own conclusions from that.