Also, if you're not police, you shouldn't be taking evidence of a crime for your own homebrew investigation. Call the cops and wait there until you see them dismantle the skimmer if you're so concerned about others.
This immediately jumped out at me. Most likely, you might get scolded or in trouble for screwing up chain of evidence or something. In a rare scenario, they might be watching the place with the skimmer there waiting for the perp to show up. Not sure of the risk there.
There was debate on how to handle it on Krebs. I think one person's suggestion of sending an anonymous tip about a skimmer to number on ATM was a good one. Keeps you out of FBI's microscope, local thugs watching don't get you, no convincing store clerks, and the skimmer gets taken care of somehow. Maybe best idea.