Well any RF equipment that you either modified or built yourself is per default illegal to use. You can buy a bluetooth-stack-on-a-chip and talk to it with your Arduino but once you sell that as a product you'll still require FCC certification.

But the FCC isn't overly concerned if you're doing WiFi or Bluetooth, their area are the broad analog strokes, correct bandwidth and correct power. As such, if you use something that has the correct filters in hardware, you'll be just fine though technically breaking the law.