This is a question to be asked on another forum as this is a Rogers community forum for help with any Rogers issue u may have.
To be honest $600-800 is what they go for. It's best to look around online via Kijiji or go to the Apple store and see what they have. I know the Apple store at Fairview mall has some used iPhone 7 devices which they might be able to sale..
Also make sure you get the IMEI number down and check if its been reported stolen or lost. If it is, do not go for it. Make sure it's not on the blacklisted as well.
Make sure you meet this person at your local Apple store to have them check it out before buying or a Rogers store. Even a police station would be the best. This way you won't have any issues.
Also make sure it's not locked to another carrier and test the device before purchasing it.