How to safely buy used iphone

You can check the status of a device to see if it's been reported stolen here:
https://www.devicecheck.ca/check-status-device-canada/
You need to have the IMEI number of the phone and you should also verify the IMEI number is for the phone you are buying.

Even if it checks out on the above web site you are still vulnerable if the owner reports it stolen after the sale as CJR mentions.

The only way to be safe on a private sale is to have the owner call the service provider where the phone was last registered and tell them that the phone is being sold to you so that the seller can't report it stolen later. Then you need to call that service provider yourself with the IMEI number and verify that it was done.

If the seller refuses to do that then there's a good chance the phone is stolen.