to hope for a donation?

Agree with you Ann and Gk
I think charity shops often have people weeding out what is valuable and what is not, and if they identify something worth £0000's it is sold through appropriate channels.
Anyway caveat emptor works both ways.
Do charities contact a donor who has inadvertently given them a Ming vase so they can have the opportunity to sell it at auction?
No, I thought not.........

Did you ever buy a house given to a charity annsixty? Quite different surely.

I wouldn't tie the two together.

I give to charities of my choice when I feel like it...so if I had a windfall, one of my favourite charities might benefit, or maybe I would spoil a family member.

When we go to boot sales, isn't that what we are trying to find, that diamond ring for a few pence at the bottom of a box of junk ?
If it was from a charity shop, I would like to think I would make a donation back, but it hasn't happened yet so it's hypothetical.

Or even give some of the huge profit back to the vendor.

But I expect that many will want to keep whatever it is they bought . They must have liked it to buy it in the first place. So they haven't actually profited.
I don't suppose anyone who bought a house some years ago and it is now worth 10 times the buying price are going to donate to charity.

Badenkate, we often say exactly that ... if it was me, I'd just have to give a substantial proportion back to the charity- and would hope most would do so.
Why is it unrealistic to do 'the right thing'? - sad.

In an ideal world yes. But unlikely. You're definitely not being unreasonable in thinking they should give some back.

Doubtful. I would like to think people would donate.

Not unreasonable to hope that it happens. Just unrealistic. Unfortunately.