Does auction house sell them verified our do they tend to go based on WYSIWYG?
Most auction houses will have someone that specialises in jewellery to provide the evaluation. They will test the stone to ensure it is a diamond and not a fake.
Here's one example of your typical jewellery auction.
http://www.bonhams.com/auctions/22633/
Edit:
Just be wary to factor in tax and buyers premium to any purchases from an auction room.