In order to deduct any expenses, it is necessary to have a company registered, both provincially and federally. I have both from my photography business and for selling new car parts and accessories, so any income is balanced with expenses, as long as I keep every receipt and kilometer driven.
If I sell my personal goods, like models, books, whatever, I do not have to declare it as income.
To be safe, I pay a professional tax accountant to do my taxes. I was audited in September, 2014 for 2013 and it took until June 2020 to clear it up (and cost me a small fortune and countless sleepless nights). You may be willing to take a chance but after 6 years of audit, I no longer take chances.
Related story...in 1985, a few of us Canadians took out a vending table for the Toledo Toy Show. We thought we could sell a few pieces and get in early to shop. Nice idea but...we were approached Saturday by a tax man who asked for our permits. We didn't have any. He told us we needed federal, state and local permits to sell as well as a customs declaration for crossing the border. We had no idea we needed any of that and got away with a written warning. Selling on ebay may have similar rules. Make sure you check before they knock on your door. :)