Users can "promote" node for a fee? Basically I want a classified ad system where users can pay extra to make their ad more visible - sticky at the top of lists, for example.

lm_paypal or pay-per-node for the simple payment for posting a classified, but how about the extra payment for making it sticky? Any ideas?
The best answer I could come up with was simply have a different node type that is automatically sticky / treated differently by views, which can then be set as a different price using either of the two modules I mentioned in my question.