SO people who gamble a lot won't share their details to gambling sites.
There is an opportunity to protect yourself and also your personal information, that you send to a gambling website. I mean the case when you only send your documents to well-known a very reputable sites, which will never sell or share your data on purpose.
What can you do is just put some kind of watermark on your scans of the documents, for example "special for casino name". It makes such marked documents less attractive to be sold, especially in case of hacking, data leaks, etc.
We will only think of selling if we think the casino is shady, but once they have license, they will not gonna do that, unless their data are hack.
For me it's not really a big issue as I am not really that important as a person, lol.. I mean if they will sell then be it, but I would never submit a sensitive information that they would know about my financial status in life.