I've never cared if my opponent openly used non-genuine cards. It's common when prototyping decks, before buying the singles, to have several cards proxied (Wo

I've never cared if my opponent openly used non-genuine cards. It's common when prototyping decks, before buying the singles, to have several cards proxied (WoTC calls these play test cards) - replaced by a representation of the card but not the actual card. There are several ways to do this - sharpie on another Magic card, a small piece of paper sleeved over a bulk common so that it's clear what the card is, or specific, sleeved playing cards. The format I typically play, Commander, is generally a casual format. My rule (and the rule of the playgroup I'm in) is that any number of cards can be proxied as long as the card clearly indicates what it is and is not designed to deceive such that we can clearly see the board state. Another format I play, Cube Draft, is like a living card game - there is a pre-determined set of cards that are drafted from. The cube I have is considered a "Power Cube", in that it contains several powerful and expensive cards. It is an all-proxy cube. Every card, except for basic lands which are added later, is a sheet of paper that is laser printed to contain an image of the card it represents, slipped in a sleeve in front of a bulk common.