I've always wondered where the line is on a product's FCC certification. Obviously US retailers such as Adafruit and Sparkfun have sold fully assembled unlicensed radio equipment for over a decade. What subtle changes could these companies make with their products that would suddenly gain the interest of the FCC?

Do you have an example? SparkFun even has a blog entry about certification that's a useful read, although not entirely accurate. https://www.sparkfun.com/tutorials/398

thanks, thats an interesting link.

That said, a manufacturer could still use an Orange or Nano Pi to integrate. Simply disable the onboard Wi-Fi and include a certified USB Wi-Fi adapter.