Is it legal to earn rewards on Buff games?

Hey guys, I was super stoked when I found Buff, but when I showed it to my friend, he thought it was too good to be true. He was also worried that it was a money-making scheme through gambling. We're not 18, and our parents would have our heads if they thought we were doing something illegal. Please can someone clarify whether this is open to under-18s and not a game that involves money?