I don't really mind those little, stray pencil marks on blank backs. If I am going to send it in to be graded (not often) I will erase them, if they can be. Otherwise, little marks don't matter much to me. This has a printed back with a stray mark on it. And it's still there.
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And sometimes you take cards with a lot of writing on their blank back because you have only seen 2-3 of them in 25 yrs.