lumydee
first the gold is a not magnetic metal, so using a magnet seek ifit fake money it is bad idea. also coin making in the meddle ages was a process doing by hand. all the coins were irregurar without a proper form. and a thougth about your vending machines, it will be easier just to provide credit vouchers that were to be used on those machine, and be selling it several places. but i believe that its too much tech about those when 95% of the people its not aware about how its working outside your city