@James Kojo - You should be giving an updated proof of funds with every offer to beat all the other who don't or won't due to "privacy concerns." If you want to buy houses for cash you want to let people know you have the money to do so. As @John Hickey said, once you show them that you are serious, they will come to you. You have to be as transparent as possible and that will help everyone know that you come through and you have the cash to close quickly.
On the agent side, when I am a listing agent and I request a proof of funds for a cash buy and there is any push back, we don't even look at the offer because a cash offer without a POF is not an offer, it's a piece of paper. As a buyer's agent for my investors, I ask them for a proof of funds up front, before I start doing all this work finding foreclosures and off-market opportunities and using my resources to help them. I need to know we are in it together and that when I call with an off-market property in their desired area with a solid ARV, they will be ready with funds to show up and close.