Let me give you an example: at BitcoinMixers.com you can find Helix light mixer that does not just clean your bitcoins it gives you brand new freshly mined uncirculated bitcoins. The Helix's system is more than a simple bitcoin tumbler, it is privacy and security wrapped in one. 
Where did you get newly minted coins?
our website is only concerned with advertising these services. where the team of BTC mixer takes new Bitcoins we do not know. maybe this is the secret of the company
So you are just advertising? I don't want you to tell where you get newly minted coins but at least can you post a signed message from an address which has some newly minted coins?
are you kidding? i said that we are just advertising
He actually has a very fair request. You "advertised" some had newly minted coins, so asking you to show this is 100 percent fair.
If you cannot prove it or show it then it should not be a advertising point.