I knew someone who did it to at least about a half dozen copies as well and had them graded. He probably ruined a dozen or so other copies at the same time. I think it is easier to do with some copies than others, depending on how the well the glue was originally applied and to what extent it has degraded over the years. I personally have never tried it and I never would, as I find it unethical. To @RockMyAmadeus point, no one ever said the Ebay seller himself faked the copy he sold, he may have bought it that way himself. Back to my original (and final) comment on the matter, I don't know why CGC grades these because it is simply impossible to tell which ones were legitimate "manufacturing errors" and which ones were homemade years later.
-J.