In terms of the stamp itself affecting the grade, it depends on where the books are grade wise.
I mean, it might keep high grade book from being a 9.6 or 9.8 and instead only be a 9.0-9.2 if the stamp is the only flaw (which can be mean it affects it lot)
If the books are in VF shape, the stamp wouldn't factor in much at all. Could lose half a grade, could stay the same.
If they are Fine or lower, I would say there is no effect really of the stamp on overall grade.
Now if who they belonged to at Marvel were to be proven (and it is someone noteworthy or that comic fans know), that could up the price. I am a sucker for neat stamps so if you do sell 'em, I'll take one with a stamp