One is that you'd need to inform the buyer about the magnet possibly erasing credit card strips. I've yet to have it happen to me or hear from anyone who's bought one.
This is thrown around a lot regarding the magnetic pens. Looking at the strength of the magnet, I did not think that it could damage a card.
I made a concerted effort on 3 different cards using both the Zen and the magnetic Graduate.
I was unable to damage them. Twenty years ago when mag strip technology was new, that kind of damage was fairly common. These days, cards are pretty rugged and very difficult to accidentally damage.