In order to play Blood Bank after purchase, you just have to create an account and you can start playing Casual! If you want to play competitively you must buy RM credits.

Real Money Credits are used in the game to exchange money between player kills. When you play competitively you play for a predetermined amount of credits, that is based on rank,skill level, and gamemode. When you kill an enemy player, you take a portion of their credits; when you die they take your credits.

In the United States of America, most states recognize Blood Bank as a skill based game; meaning it is not gambling because of the fact that your skill predominately determines your winning, and not luck. That being said, some states and countries still forbid people from playing competitively.