There are a plethora of illegal books, many run by college students, young adults and even teens. How secure are these websites??
Simple, if they're run by unknown people then why you're going to use them? you know that they're illegal so why patronize them?
What I am really asking is how easy (possible) would it be to hack the site and change a users balance without being detected??
Every website can be hacked by anybody and having the weak security that they have makes them easy to get hacked. That is why legitimate casinos that have reputations over time invest a lot in the security of their systems.
They don't get penetrated easily by the hackers but they're not 100% secure because of some updates that they've might missed or the upgrade and fault finding by the hackers through potential loopholes.