Are you looking at eBay or local buy and sell sites like kijiji, Craigslist etc.
I'd prefer local purchasing and meet at a local Telus store to insure everything is on the up and up. If the person wants to sell the phone there shouldn't be an issue.
Generally buy from reputable sites like eBay and use PayPal. If something goes wrong you'll be covered. I was burnt years ago on a blackberry curve that ended up being stolen. Since I used PayPal on eBay I was made whole again.
There are always a few bad apples online. Make sure you verify the imei with the carrier to see if it's blacklisted.