I've never asked for a proof of funds from a cash buyer before, I've always just assumed that they have had the money if they are taking the time out of their day to check the place out. Just make sure that you let them know that the transaction must be in cash. One thing you can do if you is get the buyer to give you some of your assignment fee in earnest money, that will do a lot to keep them from backing out of the deal (I learned that one the hard way by having a buyer back out a few days before closing). I hope this was helpful.
I think most investors would answer yes to Cash even if they are using hard money or private money. I've taken a little different approach to finding buyers. I have been networking at REIA meet ups and I know people that have buyers they have been working with for a long time. I'm find giving half the deal away to not worry about that side of things as I naturally build my buyers list.