There are a plethora of illegal books, many run by college students, young adults and even teens. How secure are these websites??
Whether the bookies are operated by youngsters or not, as long as they comply with everything legal to make their business legit, that's all good. As per security, no one can determine how good it is but if they are legit, just hope that their technical staff are doing their very best to make everything secure.
What I am really asking is how easy (possible) would it be to hack the site and change a users balance without being detected??
Do you mean a third-party hacker will hack the site and then change the user's balance?
If a hack happens, VISUALLY, what you see in the user's balance is as is. Users will not notice it but expect that the site's operation will freeze.