Couple Questions About Amazon Reviews

ok Im brand new here so excuse my dimwitedness.
1. Are the reviews to help increase sales of a product you are selling?
2. Can an Amazon.ca account (me) reviews an amazon.com account?
3. Do I need to have sold on amazon to be eligible to review for people?

thanks, Im genuinely trying to learn here

1. Reviews do help to improve the ratings(placement in the search) of the products. Therefore they might be reached and potentially bought more too.
2. Can you buy an item using your amazon.ca account from amazon.com? if so I believe you would be able to review them too
3. not sure