This is a sobering reminder, Stephen. Thanks for posting it!
This is alarmist rhetoric being used to sell his book. I wouldn't be surprised if he also has an interest in a supplement company he's also pushing.
Although there is some truth to what he is saying, the same issues exist regardless of where you purchase supplements, internet or not. An exceptionally cheap price should always be a giveaway.
You should be purchasing products from reputable companies if you want to have any assurance that you are getting what is advertised and what you are paying for. What few I take I buy on Amazon and have had zero issues.
In fact I am more confident buying them there (with other people's reviews and tests) that at my local market or GNC store.
As always, caveat emptor...