What about the costs of storing cash? What about the costs of going to the bank and depositing cash? What about the transport and storage of storing low denomination notes/coins so you can give change? What about the costs & risks of being robbed of your cash? What about the costs of a staff member being robbed of the cash?

Accepting cash is not free.

With cash fraud prevention is unnecessary.

Ever seen a fake note? A fake coin (yes they exist)? What about a coin from a foreign country that looks like one of your coins, but is worth a fraction of the amount? What about the costs of the little ultraviolet light to check notes for legitimacy? What about the cost of the few seconds the cashier needs to look at each note to check that it's not a fake? What about the costs of the little highlighter pen they can run across it?

Fraud preventing for cash isn't free.

Accepting it might be free, but what happens when you need to transfer large amounts of cash from your premises to a bank? Cash-in-transit services aren't free.

> With cash fraud prevention is unnecessary.

Not if we include the identification and rejection of counterfeit currency.

This does not depend on a payment method, the problem is the time delay between sending money and sending the goods - especially goods delivery not being instantaneous. So without an escrow service there will be this kind of fraud, and someone will want his money back.

[edit] And people will not only send to friends if you are successful.