  In the first part of this article, we prove two-sided estimates of hitting
probabilities of balls, the potential kernel and the Green function for a ball
for general isotropic unimodal L\'evy processes. Our bounds are sharp under the
absence of the Gaussian component and a mild regularity condition on the
density of the L\'{e}vy measure: its radial profile needs to satisfy a
scaling-type condition, which is equivalent to $O$-regular variation at zero
and at infinity with lower indices greater than $-d - 2$. We also prove a
supremum estimate and a regularity result for functions harmonic with respect
to a general isotropic unimodal L\'evy process.
  In the second part we apply the recent results on the boundary Harnack
inequality and Martin representation of harmonic functions for the class of
isotropic unimodal L\'evy processes characterised by a localised version of the
scaling-type condition mentioned above. As a sample application, we provide
sharp two-sided estimates of the Green function of a half-space.
  Our results are expressed in terms of Pruitt's functions $K(r)$ and $L(r)$,
measuring local activity and the amount of large jumps of the L\'evy process,
respectively.
