Selling a product using Arduino microcontroller or bootloader
The Arduino hardware is released under the creative commons license. 

https://www.arduino.cc/en/Main/FAQ#toc3

"which allows for both personal and commercial derivative works, as long as they credit Arduino and release their designs under the same license"

It's possible for someone to retrieve the instructions from the MCU but they would be looking at assembly rather than the c code you wrote your sketch in. 

This is a similar question and answer:
https://electronics.stackexchange.com/questions/67030/can-arduino-be-used-in-commercial-products