It can only be sold and marketed in the state it originated in or is manufactured. Another company could essentially used the same trademark if was used for another kind of product–for example, a different flavor or different ingredients–the manufacturers were not aware of the existence of the trademark beforehand, and such product is made, distributed, and marketed in another state.
Common law trademarks, therefore, can make the possibility for infringement cases to occur because their is no physical record or account of a particular trademark to ever exists; hence why it is strongly recommended to federally register trademarks. Before establishing a trademark, an individual or company must make an attempt to research that a proposed trademark is not currently being used and is mandated by United States law.
The research can be an expensive undertaking, and the time it takes to verify a trademark’s legality could be lengthy. If trademarks were all required to be registered at the federal level, such a search would not be cumbersome for a record would exist proving the availability of a particular trademark.