after reading all the arguments above, hmmm, you need to chill out, take it easy and just one thing you have to do;
How can you convince people to use your mixing service while there are some mixers around which literally better and trusted?
I doubt even someone willing to take the risk in order to use your service without any positive response and guarantee.
So you're saying you run a mixer but don't know how to sign a message?
Or if I give an address with a signature that they belong to me, will it be enough for you?
One or two addresses with the sign in message will be good enough for a start, just prove you have it.