For resales, it's true that official platforms like the one integrated into Ticketmaster offer some protection with price caps, but honestly that doesn't guarantee you the best deals. The thing is, the tickets that show up there disappear super fast, and often there's not much that's really interesting since people rarely sell at the floor price. If you really want to avoid getting ripped off, stay suspicious of regular classified ads - it's too easy to get scammed with fake tickets or transfers that don't go through.
What I recommend is checking directly with the venue hosting the concert in Montreal to see if it has its own resale platform (some venues have internal systems). Also, set up notifications on good resale sites in case new listings pop up, but be quick - good seats go in minutes. And honestly, don't just check VIP seats or front rows that cost a fortune: regular seats in a good spot in the center or in the mezzanine are often just as nice and way cheaper.