How do you make a profit selling something? It's pretty straight forward: You provide better service, quality, price, availability and innovation than your competition. Open source has nothing to do with profitability, the way you run your business does.
Arduino is a good example but there are many other healthy companies doing open source hardware: SparkFun, lulzbot, dangerousprototypes, seedstudio, TI, STmicro, I'm missing dozens more but the coffee is a little weak this morning. Mouser and element 14 even have OSHW categories that they sell. Tindie is a great place for discovering and selling shorter run, cool little open source widgets. Checkout the https://pinocc.io/ and Clyde Lamp kickstarters. My wife got her Clyde reward the other night - it's AMAZING.
How do these projects have sustainable income?
1) They sell the thing for more than it costs. Sounds simple but kickstarter is littered with projects that got this wrong.
2) Innovate to sustain (and increase) your number of customers, products, and income.
This is where the future is: I don't have the want/skills/time to build a good, aesthetically pleasing, desk lamp, so I buy a Clyde. The fact that it is open means I can change the user interface to fit my needs. Open source products will win because users will want customization that closed system simply cannot support.