The Dutch charity shops have a lot of immemorable stuff though, as you said - a lot of classical (dime a dozen), Dutch artists, German stuff, and the well known pop artists. But never anything rare.
You don't find anything rare because the volunteers at those charity shops get first dibs on everything (officially or no), and the shops themselves do; they will pick out the good record and either flog them on ebay or whatever, or move them to a more upmarket shop where the name changes from a charity shop but a vintage or antique shop.
I never expect to find anything valuable or collectable at a charity shop anymore. I mean besides the staff, there will always be people scouring it at all hours of the day looking for collectibles.
The other one there is house clearings; in some cases, when someone dies with no relatives or nobody interested, they'll hire a house clearing service, who also get first dibs on anything valuable or collectable. The rest is sold in either their own shops or sent to a charity shop.