No clay will not remove marks or restore dullness. But, if you haven't clayed your car then the previous owner probably didn't clay it either. There are great advantages to claying your paint. I clay my car at least once a year to remove contaminants that waxes, washes, and polishes don't remove. A test to see if you need to clay is to put your hand in a ziploc bag and lightly run your fingers over the surface (without moving the ziploc bag, just your fingers) If you feel any roughness, you need to clay. You'll be surprised at what a good (1) wash, (2) clay, (3) polish, and (4) wax will do to your car. [sm=smiley20.gif] Makes a world of a difference.
Edit: Just use plenty of lubrication and fold the clay to a new portion once it gets dirty. If you drop it, throw it away.