A difficult problem. As soon as you do an exchange in person you can be discovered as the police can always pretend to be buyer/seller. Web of trust comes to mind, but it would be difficult to bootstrap it, and the community should be large enogh and decentrilised so that arresting few people cannot distrupt the exchange.  

 You are right. But those people have no chance to get banking. So or you risk your life to do with cash transactions, or you die without money. No choice.