I really don't know how well one can protect them when they click on anything that moves..lol Just be aware of what your teen is doing.
You don't don't need to be a "vulture" perched above him while he surfs, just be mindful.
A good virus program should help, Avast is what we use and it sets off a siren if a virus or a Trojan worm sneezes near the computer.
With the invention of Craigslist and other sites like that the scammers always find a way to make it hard for legitimate sellers to use the site.