That’s going to be a tough one as you no longer have the camera. Does the buyer know how to use a P67II and has read the manual? Is there film in the camera that is loaded properly, or is he dry firing? My first guest would be user error as the camera was working fine when you had it.
However some 67II users have experienced the same stuck film advance lever, so it’s a known issue.
A Pentax 67II can still be repaired by Pentax themselves, though you may have to ship it to Japan.
International Service Network (Japan) : Support & Service | RICOH IMAGING
Phil.