Import marks are an unfortunate government regulatory requirement and a regretable necessity for many weapons.
I have a very small import mark on one of my non Luger pistols. The particular pistol is a beautiful work of art. But, the import mark is a mood killing distraction....like being distracted by a giant pimple on the face of a scantily clad Scarlett Johansson.
My personal opinion is that an important marks reduces the value anywhere from 15-40% and is dependent on the mood of the buyer and the rarity of the gun.