A variation of this has been proposed before. However, it wasn't an auction. The idea was that people could include a micro-payment with their message. If their payment is below your threshold then the message would be bounced. If the payment is above your threshold then it is delivered to you. If you like the message then you refund the payment. If the message is spam then you keep the payment.
This system makes most spam uneconomic and financially compensates users for the remainder. It can be combined with whitelisting and it doesn't preclude legacy inputs.
It is certainly better than Microsoft's digital stamp which earns you nothing and Microsoft one cent for each of your messages - legitimate or not.