this is a very old topic and also a very current, present day one.. Solutions exist that may not seem obvious in the modern context, because the world you and I live in, is so different than the past.

In my own experience I have negotiated sales or contracts with others for non-trivial sums. It feels like most modern people are just going along with systems that require an intermediary and working devices. I feel it is not only important, but crucial, to have the ability to negotiate directly with a peer or others. That means without electricity or cell phone coverage, too. YMMV