You don’t. That’s what bartering is. The only thing that cash does is gives you a baseline for bartering so that if the two of you don’t directly have something to exchange otherwise, he can use your cash to get a service he needs elsewhere.
As long as the cash is available, the two of you are able to have an exchange.
If another payment processor is involved, that freedom changes dramatically.
1. You must have another means of transacting, bank card, etc
2. Seller must have a way to process your transaction and hopefully his way accepts what you have to offer (Visa, etc).
3. The way to process the transaction must have some type of connection available to report success.
4. If for any reason the payment methods involved don’t think one of you should be allowed to proceed...it denies the transaction.
Ultimately, this adds a lot more friction to the process and requires someone else give permission before the transaction is allowed to proceed.
Cash itself is just streamlined bartering. No 3rd parties means no 3rd parties involved in allowing the transaction to move forward aside from the 2 involved.
And you can turn around and use that cash in another cash transaction, and so on and so on, forever...without another a bank or anything else ever being involved.