Yes, the coefficient of friction is always a positive number. The friction force always opposes the direction of incipient motion. If you are pushing the block up the ramp, the friction force acts down the ramp, and vice versa.
That's a strange way to describe the coefficient of friction, to say "When this occurs the friction force is F=μN".
That is always the case, not just in the limiting case of the angle of repose.
Anyway, on the way to deriving the final inequality, you should make use of this relation, that tan α = μs the static friction coefficient