I'm pretty sure you would need a USDA license to sell plants. I know plenty of people don't get one but if you get caught there's a large fine and I believe they can confiscate your plants as well.
Now, speaking as if I were driving down the road and saw someone selling orchids roadside, I would stop to take a look. The only way I would buy any is if they were dirt cheap-$5-$10 is all I would pay to someone that:
1. I can not get a hold of for care info.
2. I can not take it back to if there is a problem or it dies.
3. has no overhead.
I am inclined to spend more money with a nursery that is reputable.
With that being said, I think you could make a few bucks selling roadside. Just be prepared for people like me to haggle with you over price.