Yes, foreigners can freely purchase and board plane tickets from Hawaii to the U.S. mainland.

Since Hawaii is a state of the United States, flights between Hawaii and the mainland are classified as 'domestic flights', but foreign travelers can use them without any restrictions as long as they have legally entered the U.S.

To explain in more detail, when a foreigner enters Hawaii, they go through the usual U.S. entry procedures (ESTA, visa, passport inspection, etc.). In other words, by the time they arrive in Hawaii, they have already passed through U.S. Customs & Border Protection. After that, they can move freely anywhere within the U.S. Since flights from Hawaii to San Francisco, Los Angeles, Las Vegas, New York, etc. are all domestic flights, there is no additional passport check or visa inspection.

Purchasing plane tickets is also simple. Foreigners can buy tickets directly on the airline's official website, through travel agencies, or at airport counters using a credit card or online payment. There is no need for a U.S. address when making the payment. However, there is an ID check procedure when boarding. Foreign travelers without a U.S. ID can present their passport. With just a passport, they can board just like U.S. citizens.

For example, if Japanese, Korean, or European tourists arrive in Hawaii, stay for a few days, and then think, "Shall we go to LA next week?" they can book a flight without any issues. In fact, many foreign travelers continue their itineraries to the U.S. mainland after traveling in Hawaii. There are dozens of flights operating daily from Hawaii to major cities on the mainland, connected by various airlines such as American Airlines, United Airlines, Delta Airlines, and Hawaiian Airlines.

There is only one point to be cautious about: the validity period of the visa or ESTA must cover the entire trip. While there is no new entry inspection when moving from Hawaii to the mainland, the already granted stay status (ESTA 90 days, visa duration, etc.) continues to apply. In other words, the day of entry into Hawaii becomes the start date of the stay in the U.S., and even if they move to the mainland, that period does not automatically extend.

In summary, once a foreigner has entered Hawaii, they can freely buy and board plane tickets to the U.S. mainland. Traveling from Hawaii to the mainland is not considered 'international travel' but rather inter-state travel within the U.S. All that is needed is a valid ID (passport) and a boarding pass. Therefore, many travelers choose to use Hawaii as their first gateway before continuing their journey to the U.S. mainland.