When I was in high school (Early 2000s), a guy I knew used to do this with boosters for Yu-Gi-Oh. He'd sell open booster packs, claiming he didn't change with the contents and only opened them to see if he wanted to keep what's inside, but I never believed him; if you're already opening boosters, what's stopping you from altering them in ways that benefit you?
I tried explaining this to my friends who bought from him. I guess that's why they played Yu-Gi-Oh, and I played Magic (Yu-Gi-Oh from that period isn't nearly as complex as it is today; there weren't even limited or banned cards in tournament play yet).