I know it can be used for CMS. Can MT be used for creating a good classified ads site?
Which publishing type is good for a site like that? (Dynamic or Static)?

This is such an open-ended question that any answer will be equally open-ended. That said: yes, you could use MT to create a good classified ads system. IMO, a well-crafted static-publishing system is pretty much always going to be a better performer than dynamically-rendered pages. Though to be fair, I haven't used dynamic publishing much.