Privacy is the ability for an individual or group to seclude themselves or information about themselves, and thereby express themselves selectively.
I've been cashless for about a decade now. I have no problem with donating money, giving or recieving monay as a gift. I never give the homeless money, but I often buy them food. Why couldn't you buy or sell something to people? You can easily transfer someone a small sum of money using their phone number. Same with garage sales. When was the last time You saw a Piggy Bank?
Some people already live in a cashless (and moneyless) society already - the homeless and those who are incarcerated. Also children. They tend to barter and trade for stuff. It's not that crazy of an idea, most people figure it out.
Some restaurants deliberately stopped accepting cash to exempt homeless people from patronage. Imagine being so gross that you change your policies to bar people from getting food.