I have this camera and lens, and, while cruising Youtube, have seen several good videos on how good that lens is, which backs up my experience.
I've used the one I have, as a stopped down lens on Canon F1s n&N, and now Nikon, because of the nice rendering, at about F5.6 - F8, which is different than what the Canon and Nikon lenses give.
I feel the FD Canons are crisper, in b&w and the two Nikon lenses I have An 'E' and an Ai, in that range are colder, while the Pentax 55 is warm and more smooth.
I have no shot colour films with the Nikons or Pentax 55, but it's liked by more than a few, online at least for it's colour rendering.
I am beginning to love M42 lenses, because of the fact alone that there are so many good adapters out there and using a stopped down lens, forces 'depth of field' use, which many new photographers of film cameras do no bother with, only match the arrows or line and circle, and shoot!
Good luck with your sale.