Head or tails with real money, thats quite a concept there . Either win or Loose. 50-50 chance. Its gambling so very much legal if your country supports gambling as legal. Taxation will however be mandatory i guess, as earning from gambling invites tax in most countries.
That is gambling, pure and simple. The laws of various countries or states that prohibits or regulates gambling may apply, including where you are based, where the site is hosted and where the players reside.