While the reality is much less romantic than the concept, there are people who make a living doing just that. Look at Arizona, where there is a huge seasonal trade in both gemstones and finished jewelry. People work all year to sell at those big shows, and they make enough to just do it once a year.
Others travel all over, selling at Renaissance fairs and craft shows. There are books published on the circuits, so you can plan your route from state to state.
Being successful would mean making a lot of sacrifices, and being willing to live on little during lean times. You would need a lot of self-motivation and the ability to save for rainy days, but to me, the benefits would be well worth the sacrifice. Imagine the things you could see and experience on the road, from national monuments to hidden treasures.
While being a traveling jewelry merchant is probably not the life for me, it sure does have its appealing points. Maybe someday, when the kids are grown and life is more settled…