Not sure I understand your perspective from your question but i will answer best I interprete.
The person who did not pay me was an individual aquaintance who asked me to buy some nutritional items for him. I gave him the products and he said he would pay me back later. All very informal but then when I asked him for payment at a later date he claims he did pay me.
Nothing was in writing because one does not supspect this would happen then as the other answers to my questions suggests "put it down to one of life's throws". The moral is don't be so trusting.
I wonder if it is possible to take him to small claims regardless of outcome?