My ds's school have a really good secondhand uniform shop, run by the parents association, who get all the money from it. They used to do it so parents donating clothes got a percentage, but that was really way too complicated, so now it's just a donation, and a good fundraiser. It opens 4 times a term, plus the spring fete - this year there were lots of prospective y7 parents shopping in there, and over the last 3 times it's been open (spring fete and 2 days after half term) we made over £700.
We also go through the lost property once a term, return anything named, and wash and sell anything unnamed that's in decent enough condition.
You need a room where you can keep everything though!