Alternatively, both people could have yelled out to the surrounding village that you owed that person a banana. Then you didn't need the seashell. Everyone knew you owed that person a banana. Then you go back to your hut the next day and get the banana. Then you yell out to the village that you paid back the banana. Let's hope that banana did not go bad overnight.
Would it be possible to have a computer store these transactions so the villagers wouldn't have to remember?