Investors remain nervous after the massive sell off in February and March, but Matthew Kidman from Centennial Asset Management says that any sell off (such as we've seen in the week since this was recorded) should be treated as a buying opportunity."If the market does come away in September, use it as a buying opportunity. Now is not the time to get off the train. Now is the time to buckle in, ride a few bumps out, and we're gonna go again."In this episode of The Rules of Investing podcast, we discuss the curious origins of Buy Hold Sell, why investors should treat any sell off as a buying opportunity, and we get his view on a range of Aussie small caps, including OohMedia, Adairs, and iCar Asia.