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Originally Posted by Doug McClasky
is that like an object floating underwater that has water acting against it but has buoyancy acting to make it rise so that it's stationary but there's still a force trying to make it accelerate?
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No, in that case the forces would balance so that the acceleration would be zero. The point with the ball is that its velocity is changing, even when it is instantaneously at rest at the top of its motion.
If the ball were sitting at rest on the earth, however, then its acceleration would be zero. Even though gravity pulls down on it, there's a corresponding upward force exerted by the ground that balances it out.
Being "at rest" (i.e., zero velocity) with nonzero acceleration only happens instantaneously.