Here's a reflective, full confession from a kid of the 70s who is happy he still has his cards from back then (and more, now)...
I loved getting checklists, and I marked them when I got them. That's what checklists are for, after all. Even today if you want to know what cards you have or don't you go to the TCDB "checklist", right? Remember there was no internet then, or even a Beckett's or Topps magazine with a full list of cards. But those occasional team cards or series' checklist cards would tell us who we were missing.
Somewhat ironic in hindsight is that it was the doubles and triples that didn't get marked and those were the ones I traded to friends for whatever cards I was missing. Today those are the ones I wish I had.
But what I really lament is that I always used pen or pencil, which of course makes an indentation in addition to a mark. But the pencils faded over the years and at this point I really wish I had used my Dad's fine-tip colored Flair pens (markers?) he had instead of what I used. I think a fine-tip Sharpie would have been ideal, wouldn't have indented and likely wouldn't have faded. But I can't say I ever saw such a thing back then. (Erasermate pens were all the rage, though, when those came out!)
Next time I'm in a tricked out DeLorean and heading to a 1970s five-and-dime store, I'm taking a Sharpie or two with me for pre-teen me, should I run into him.
--Kent