www.bigfootcomputers.com Best mod and cooling store out there. They don't always have the best price but service is worth every penny. For hardware such as HDD and Motherboards I use www.NCIX.com also have good service.
There's a thread about canadian online vendors in the vendor discussion forum right here . Really it depends on where you live. Some of the stores in Ontario will charge you PST whether or not you live in Ontario, and probably give you SARS (just kidding.... ) Some stores that have real storefronts (like NCIX for example) are an especially great deal if you live in the area. And for checking the lowest price on items in Canada, just go to pricenetwork or computers canada .
Hope this helps
EDIT: And as for selling old parts, all you need is 100 posts here and you can post in the buy/sell forum. (or at least that's what I hear )
I'm not a big fan of Bigfoot for the way I was treated on my very first order from them. Took them awhile to ship my order, even thought they claimed it would ship same day. Then me and teh manager got into an argument over it and I found him to be a very arrogant businessman.