Ideally if you bought something from kijiji you communicated by email with details or just print the ad you replied to that gives the item and price. Then on that put the date you picked up the items and how you paid - since most expect cash. In the event of an audit with the picture you could produce the item in question. You can do the same with a garage sale item to take a picture of it and record day and price paid but just writing down a description of the item, where bought from and the amount is good enough.

On the opposite side of the spectrum is things like a bill from the dollarstore or similar that just gives totals and not what the item actually is. Write on the back what it was - either item name or catagory ie craft supplies.