Are certified gold coins the only type I should consider purchasing online?

While certification from top-tier grading companies such as NGC, PCGS, and ANACS has definitely added to the safety of purchasing rare gold coins over the Internet, uncertified ("raw") coins can also be acquired online at a good value if you're provided clear photos of the coin(s), have acquired the knowledge to grade accurately yourself, and if the price of the coin is below your personal risk threshold.  

And remember, just because a coin listing shows a photo of it slabbed by some grading company doesn't remove all risk...there have been rare reports of slabbed coins in the marketplace that are actually counterfeit.