I do not know about items of great value, but we are auction haunters, buying and selling bits and pieces, mainly on ebay. I paid £50 for a box of 10 bruised and battered children's sewing machines and while most were branded Vulcan and sold on ebay for £10-£12 each and several ended up in a charity shop, one was a French make and came with a box and sold for £120. DH bought a box of old fashioned fizzy drink bottles. Most sold for £1 - £3, but one sold for £70.
Last week DD took an old cow hide rug to an auction. It had been on her living room floor for about 15 years and was badly worn. She paid £10 for it in good order and sold it, shabby and worn, for £50
There are a few people who pick up eyewateringly valuable items for £1 but not many, but what we have learnt is just how much you can get for quite unconsidered trifles lying around the house.