I can prove ownership by uploading some custom html.
That's just a page. Anyone can code simple HTML snippets. You need to have at least a working product to have people invest for you. You need to prove you are worthy of operating and managing a gambling site. It's better to ask people to invest in the bankroll.
There are many dice sites and the competition is tough, if you don't even have money to start a simple site, you won't have any success in this business.
I'm not a pro on this but he actually invest something on that domain. Maybe around 2$ for the domain it's name is not that premium. I don't know if he avail already web hosting since that will cost a lot. But the most important part is the gambling script if he had or not. A working for a gambling website is very easy once you have script which is the expensive part.
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Hey OP. Do you have already script for your dice/casino? If none. Then get lost if yes then we can discuss on PM about that.