Yes you can. If possible write a receipt yourself on a piece of paper with a date, price, name of item and where you bought it and ideally get the person to sign it when you pick the item up. If they won't sign still just keep the piece. I am assuming if it is off kijiji or something like that that you had to email the person so if you have an email that details location and price, print that out as your receipt - all you need is something that proves you bought it and how much you paid. You could also right click on the ad from kijiji and copy and paste it to a word document and that would show price. But the email idea is the best.

For those just starting out you can do a similar thing with things you have for your own kids that you are no longer using such as a playpen or highchair. You basically sell it to your daycare as if you were selling it at a garage sale and a daycare provider bought it. Try to pretend you are two people doing the transaction. Scout garage sale ads to get an idea of the going rate (usually 30-50% of the original price depending on age and condition of the item).