Can I buy a plane ticket with my passport if I enter the United States illegally?

You can't. If you have a passport but enter the country illegally, your passport will be invalid and you will be detained and bear legal responsibility. Therefore, you can't buy a plane ticket with your passport if you enter the United States illegally. Passport is a legal document issued by a country to prove the nationality and identity of its citizens when they enter and leave the country and travel or stay abroad.