Pulling it isn't the issue. It's supporting the pin weight of the trailer. By the time you add people, cargo, hitch, and accessories to the truck there won't be much payload left. I don't know which trailer you're looking at but likely it would overload the truck.
A few things to keep in mind:
Salesman generally will say anything to make an sale.
He is correct, it will pull it. That's not the issue.
A trailer that is "1/2 ton towable" doesn't mean you can take any "1/2 ton" vehicle, load it down with whatever you want, and then still pull the trailer.
If your truck is actually a regular cab 2wd model with no options and there will only be two of you in the cab with no other gear then you could probably be withing the payload rating of the truck. I highly doubt this is the case.