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Let me think about your perspective's online application : if you are good in observing then you may get chances to guess the seeds and then you may bet accordingly. It is not easier as it sounds but I heard someone guessed (probably might have hacked) some house's seed generation sequences and made 2000+ bitcoins.
Guessing seeds is technically impossible even for the most powerful computer in the world, let alone human mind. The case you mention in your last sentence is about HufflePuff hacking PrimeDice, but he didn't "guess" the seeds. He exploited the situation when several accounts were sharing the same server seed, and although he didn't know the seed itself he knew when he's going to win and when he's going to lose, and he wagered accordingly.
To OP, be cautious! Firstly, it can be an illusion that you know how to cheat the place. Secondly, cheating in places like this can be very dangerous.