Nice effect!
I enjoyed it very much. You could follow up with taking your coin back from the spectator, and then place the spectators coin in the spectators hand, have him close it as before. A slight rub on your coin and it vanishes. The spectator finds your coin, but you point out there is only one coin, he flips it to find his own signature. Yours on one side the spectators on the other. You bring out the sharpie for examination and you are completely clean.