Give it the grade you think it deserves, and stick by it. That's the grade on ebay, that's the grade in a sales thread on the Boards, it's the grade in PIF/TGT, it's the grade in this thread...
Then you can have whatever kindly return policy you want.
+1
What Speedy said. If all else fails, grade it, drop it half a grade to be safe and move on.
I don't like the sound of 'grade it whatever you want and have a kindly return policy'. That's not how it is supposed to work. I would simply say tighten up your grading. Don't give it the highest grade possible, give it the safest grade possible.