I think having a second hand shop in Diagon Alley could be awesome! It could either work like all the other current shops, except the things would be of worse quality (having been used) and be cheaper, for the witches and wizards with less money.
Or, maybe students could sell their things (from the chest) to the second hand shop for others to buy. This way you could get rid of things you no longer needed, but get a few coins in return - and others could buy it.
The second part is less important, I just think having a second hand shop would be really cool, besides there is a second hand shop in Diagon Alley in the books!
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