From the groundup how to betting sites create a bankroll before the site opens?
If you have to ask...you don't got the funds.
Should you have the funds...you should check out a trusted, already proven top bitcoin gambling site like
https://bitcoinvideocasino.com/, which is for sale
If you have to ask you do not have the knowledge, who says he does not have the funds and is pondering what to do with it?
Buying a casino is a whole different game, it requires more funds than starting a new one from scratch, but the time of ROI might be shorter and the income and userbase more stable.