I just rewatched this moment. Kaiji takes 3 citizens and put them aside, take a citizen and a slave and put them on the table, then put citizen and place it on top of the slave. After that, I think, he takes the top card (citizen he just put in), Tonegawa sees it, Kaiji marks cards with blood. So marked cards are indeed those he used in 11th game - a citizen and a slave.
Hply crap, Kaiji managed to make a trap inside of a trap! He is indeed Akagi-like!