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Yeah casino bankroll works.
But that doesn't go without risks :/
First risk is of course to see a big player getting lucky
But you also got the risk of seeing the casino owner run with your profits! And that's not something you want xD
Meh. No such thing as risk-free.
Even simple Holding (short/long term) is not without risks.
Hence the primary "work" needed with regards to casino investments is choosing where/which.
(This isn't much of a concern for me personally as I already have a definite idea of the 'where/which'.
)
In my opinion there are no such investment opportunities in bitcoin unless those investment sites that turns into scam later on. Cloudmining sites also turns into scam. I wouldn't consider investing on a casino's bankroll since there is a great chance that you will lose money here if a player got really lucky. It is worth a try though since you might earn passively.
There is always that chance.
Again, that is why choosing where/which casino to invest in is critical. You'd of course, best choose one where their bankroll management is top-notch. (ie. Max amount a player can win in a single bet should only be a certain percent of the bankroll. )