As a national cash buyer of houses, we can provide insight into the other sie of the equation.
With a network of thousands of investors across the US and Canada, http://WeBuyHouses.com
is one of the largest cash buyers in North America.
Cash buyers are VERY different than a more traditional home buyer who might be financing the transaction.
When you have a client that has selected you to work with and is high net worth or a professional investor, asking for "proof of funds" is one of the ABSOLUTE WORST things you can do! Successful people expect to be respected with a certain level of customer service, and that type of interaction is disrespectful and sets the stage for a lack of trust.
Imagine you went to buy a luxury car, and as soon as you walked in and asked to test drive a vehicle, the dealer wanted to pull credit and see your bank accounts to verify you could afford the vehicle. I don't know about you, but I'd walk out of the dealership.
Certainly do some research (the Internet makes this very easy) to verify they aren't being deceptive, but if it's clear that someone is likely to be able to make a cash purchase, don't insult them by asking for verification.