It is not "unprofessional" to use a ThemeForest theme.
What does that even mean, really? Break it down.
Do you think customers will not buy if you use a ThemeForest theme or Twitter bootstrap or WooThemes or what have you? If so, empirically denied: I've sold hundreds of thousands of dollars of product on sites using those options personally, and many people have had that experience.
Do you think you owe it to the universe to hire a designer? I don't know why you'd think that, but some people apparently do, so let's time-shift that owning-designers-money thing into the future, just like designers have apparently timed-shifted giving-you-more-than-$50-for-your-programming services into the future.
Do you think "professional" sites necessarily have awesome, jawdropping unique design? That is not true and would not withstand scrutiny through the WayBack machine. The biggest Internet companies you can think of all launch withed worse than what $5 will buy you on ThemeForest. Some of them are still there (cough Craigslist).