According to the United States Customs and Border Protection (CBP), in order to import goods for commercial purposes, valued over $2,500, you must post a customs bond ensuring all duties, taxes, and fees owed to the federal government are paid. This also applies to a commodity subject to other federal agencies requirements (such as firearms or food).
An importer can use a customs broker to clear the goods through CBP, relying on the broker’s bond to secure the transaction. Customs brokers are licensed by CBP, and are authorized by U.S. law to act as agents for importers. Customs brokers prepare and file the necessary customs entries and other paperwork and obtain the CBP bond.
While a convenient option for importers, there is no legal requirement to use a customs broker when importing to the United States.
So what is required to import into the United States?
If an importer opts not to use a customs broker, the importer must be educated on the CBP’s many policies and procedures as well as the entry requirements specific to the particular commodity being imported.
The importer must file entry documents with the port director where the goods are coming in. Imported good are not legally entered until after the shipment has arrived in the port of entry, delivery of the merchandise has been authorized by CBP, and estimated duties have been paid.
The Importer is responsible for arranging the examination and release of the goods. Importers are also responsible for contacting other relevant agencies besides CBP, for example, if the importer is bringing in food, the Food and Drug Administration must be notified.
In some cases, the importer may be required to obtain a permit, license, or other certification, depending on the good imported and/or the policies of local or state authorities which, may require a license in order to do business. 
As mentioned, a customs bond is required when importing merchandise for commercial purposes that are valued over $2,500 or a commodity subject to other federal agencies requirements (i.e. firearms or food).
Importers must hold either a single entry or a continuous customs bond. A continuous bond is recommended if importing frequently and through various ports of entry. To do business with customs using a continuous bond, importers must apply for permission through the entry office at the port where the goods are imported.
If importers fail to file all of the paperwork or properly follow each CBP procedure, they face major fines and risk seizure of the merchandise. Importers operating without a customs broker, must know the requirements of each type of import to successfully import their goods. According to Vanderbilt University’s Procurement Services, while importers are not legally required to use a Customs broker, few businesses or organizations:
Have the human or financial resources to keep pace with the constant regulatory and technological changes throughout the world which are specific to each country involved in the international shipment of goods.
A customs broker is familiar with the Tariff Schedule, the list of duty rates for important items, and the regulations governing importations (found in the Code of Federal Regulations vol 19, ensuring the importer’s goods are cleared without any costly delays or fines.
Again, it is not required to use a customs broker to import to the United States but importers might, at the very least, want to consider using one for difficult-to-import products such as cosmetics, foods, live animals, and vehicles.
[1. U.S. Customs and Border Protection. (2015). When is a Customs Bond Required. Retrieved from https://help.cbp.gov/app/answers/detail/a_id/734/~/when-is-a-customs-bond-required.]
[2. U.S. Customs and Border Protection. (2006). Importing into the United States. Retrieved from http://www.cbp.gov/sites/default/files/documents/Importing%20into%20the%20U.S.pdf]
[3. U.S. Customs and Border Protection. Tips for New Importers and Exporters. Retrieved from http://www.cbp.gov/trade/basic-import-export/importer-exporter-tips]
[4. U.S. Customs and Border Protection. (2006). Importing into the United States. (Page 11).]
[5. (2006, Page 11).]
[6. U.S. Customs and Border Protection. Tips for New Importers and Exporters.]
[7. Vanderbilt University, Procurement Services. (2009). International Shipments: Customs Brokers, Why and When to Use a Customs Broker. (Page 1).]
[8. (2009, Page 1).]