Product Name: Ultrashocking Dark Mark!
Product Description: Have you ever dreamt of getting back on someone who once conveyed a terrible fright upon you? Have you ever wished to scare one of your enemies so much that they'll leave you alone forever? Or do you simply wish to delude your parents? Well, this day,
your day, has finally come! Buy the Ultrashocking Dark Mark from WWW's, and your dreams will come true! Simply place the mark in the attached Glutinity Liquid for a few minutes, then place it on your forearm and it will stick to you in an almost frightening manner for up to 10 hours! Here come your 10 hours of deception!
Product Placement: Weasley's Wizarding Wheezes
Oops, I just realized that "Edible" Dark Marks already exist!

'Cause I just started re-reading HBP.