There are a plethora of illegal books, many run by college students, young adults and even teens. How secure are these websites??
What I am really asking is how easy (possible) would it be to hack the site and change a users balance without being detected??
So I assume that you mean sports book right? since you are in gambling section so maybe that site is what you are referring on.
But since as you mean its illegal then its been operated by college student or young adult then it does mean that those illegal book makers cannot pay their obligations to their site users especially if there player gain something from their site.
So much better not to use it for risky reason and much better use those sites that have reputation and also have good result for compensating their site users when they earn something from them.