Are all cranks with round tops metal? Are all cranks with flat tops plastic? I've seen eBay listings showing round-top cranks said to be plastic, and flat cranks said to be metal.

Yes, all round top (dome) cranks are metal. Some metal cranks have triangular arrow head pointers and some do not. Older cranks do not have the arrow head. Very old cranks are slightly lower in profile that newer cranks. The crank arm is therefore lower, which often caused an interference problem with the clearing lever plunger as the clearing plate was spun around. The arrow head on metal cranks is a triangular shape outline. Some arrow head outlines are filled with white paint; some do not have the paint.

On the Type I, the subtraction band on the metal crank was always unpainted bare aluminum. On the Type II, most metal cranks had an unpainted clear aluminum subtraction band; some later metal cranks did have a red (or even fluorescent orange) subtraction band.

To answer your second question, yes, all plastic cranks have a flat upper surface. Also, all plastic cranks have a solid arrow head pointer and a red subtraction band. The solid arrow is molded into the plastic and filled with white paint.

The eBay listings you mention have an improper description of the crank material.