The Estirator shows recent item listings from eBay Kleinanzeigen to the user, without letting him know the price. It's the users task to estimate a price for the items.
When the user submitted his estimations he gets access to a list of items, sorted by how much under worth they are sold.
Advantage for users:
Find super cheap offers within seconds.
Advantage for sellers:
Get an idea of how much customers would spent for your product.
Advantage for eBay:
Possible A/B testing for product photos and their influence on the customer.