If you have a large warehouse full of overpriced stuff, then on average something sells every day or so for a good price, it is a good business. You seem to find it in every business, I wanted a bucket ram and links for a digger, £1800 plus VAT! No guarantee, still got them years later.
On the other hand I sometimes wonder if it is a scam. Look at the prices of ordinary secondhand books, from £5 to £250, same average condition. But if you want to shift a lot of cash from point A to B then what better way to do it? Get an item, with an invoice.