"CWTA recommends consumers always acquire wireless devices from a trusted and reputable source." = buy from ROBELUS and let them make profit.
*Reply for Goran's comment
I sell phones on Kijiji and LetGo because there's a market for it.
I'd never buy phones used given the risk of it being blacklisted after the fact. This is especially a big concern for a $600+ phone (mid-range to premium/flagship), which sounds like the type of phone OP is going for.
For higher end phones, Goran has provided awesome options, and Ivan praises powerful phones from reputable Chinese brands that sell for a sliver of the price of an in-market flagship.
Alternatively, you could have a family member who's 18/19+ grab a Koodo phone on a post-paid account and then sell it to you, or have you pay the bill.
Figure out your wishlist of the phone you want (and fallbacks, if you can't get a good deal on the phone you want), and find the cheapest option to get it. If you're into LG's, they tend to sell for cheap on eBay. Samsungs and Apples are less easy to find at a discount.