To sell your old smart phones, you would either sell it on http://www.ebay.ca or http://www.kijiji.ca.
(1) With Ebay website, you will list your item for sale as auction style and once someone buys it, you will need to ship the item to that person's address. You will have to sign up for both Ebay and Paypal services (Signup is Free). The buyer will pay you through paypal and you would notice that your paypal account has been credited with the sale amount. Ebay and paypal both takes a small fee for the sale transaction and the remaining money is for you to keep. A piece of advice for you, always try to use the shipping services where the receiver of that item will have to sign once he/she receives the item. The reason why this is important is that the buyer can later argue that they never received the item and then whatever money you earned will be refunded back to the buyer and you will be left without the item and without the money.
(2) With Kijiji website, there is no payment gateway as such like paypal. Most transaction takes place with money and face to face with the buyer in some local area of your choice. This type of selling is harder because both you and your buyer has to see when you two are available and then find a public space to meetup and exchange item and transaction in cash. It is also more risky and dangerous.