Not only casino sites but other sites can also sell user documents if it is a scam site. But the real sites and those who come to do real business will never sell their users' documents. Because it's an important thing. Which can be used to complete various types of illegal work. So real sites will never play with the security of their users
Users don't hesitate to send their kyc documents on real websites. But a website is only real until it scams users.
I would say that there are always pointers through which we can know which site is fake, but most times, we are so in a hurry of missing out on something that we get blinded from seeing those pointers.
To ascertain whether a site is real, how long they've been in business, how they run the business and how they treat their customers should be a great way to find out, plus other stuffs like if the company is genuinely registered , if they have operating licenses and if those licenses are genuine.
A real website will have all this documents, and every needed information in place for verification of those that need it.