From deeper in the thread, a follow up email:

Hello Edward,

Thank you for taking the time to speak with me. As we discussed we received information concerning some questionable or undesirable activity that occurred at your store. We understand many of these issues stem from misconceptions or miscommunications and I have summarized what we discussed below.

• Magic events (sanctioned and unsanctioned) can only allow genuine Magic: The Gathering cards. • Proxy cards are substitute cards created solely by judges in sanctioned tournaments pursuant to the official tournament rules. These substitutes are allowed when authorized game cards become unplayable during a sanctioned tournament because of damage or excessive wear. • Counterfeit cards are copies or reproductions of actual Wizards trading cards, whether or not they are identified as non-genuine. The creation and distribution of counterfeits violate United States and international copyright laws and negatively affects the integrity of Wizards’ trading card games. Counterfeits are strictly prohibited, even for personal, non-commercial use. Like · 1 hr

Matt Ling Matt Ling Here's a little rules lawyering for you: from my reading of this, it's not clear to me that it is against WOTC policy to run a non-sanctioned event, but with cards written with a sharpie. That's not a "counterfeit" card (it's neither a copy nor a reproduction), and the cards are actually "genuine Magic the Gathering Cards."

You can just have agreements between players that the card represents something different. E.g., that the "Doomed Traveler" card that I just played actually has the same effect as a Mox Opal.

I don't think WOTC is empowered to tell me how to play Magic in an unsanctioned event.