I urge all sellers and would be sellers (located in Ontario) to invest the time to review the Retail Sales Tax Act of Ontario:
http://www.trd.fin.gov.on.ca/userfiles/HTML/cma_3_2491_1.html
Sellers (and would be sellers) residing in other provinces should do the same with their respective provincial government sales tax department websites.
All Canadian sellers (and would be sellers) should also invest the time to learn about the advantage of registering for GST/HST by visiting the GST website:
http://www.cra-arc.gc.ca/tax/business/topics/gsthst/menu-e.html
For more useful tips, please take a look at tax information for small business in Canada:
http://sbinfocanada.about.com/od/gsthst/index_a.htm
Finally, do yourself a favour and read all of it, not bits and pices here and there. Do not select a sentence out-of-context because you like what it states.
To understand the tax system is part of doing business in Canada. If you want to sell on eBay (other than selling personal used goods) you are in fact becoming a small business. To understand the system will save you money and time on the long run.