purchases from private persons

I would like to start a webshop selling second-hand clothing. I would buy the items at flea markets and second-hand fairs. Often, the amounts involved are small, such as 1 or 2 euro per ‘stall’, i.e. from private individuals. Do I have to create a separate proof of purchase for every item of clothing costing, for example, 1 euro? What if the person refuses to fill in this proof because it is ‘only’ a few euros? How can I solve this?