Before re-entering the U.S., consider what you have purchased while abroad and whether or not it is legal to bring these purchases back to the United States. For example, fruits and vegetables, seeds, drug paraphernalia and certain food products such as cheeses are prohibited, as are ivory and tortoise shell products. Medications purchased abroad may also not be FDA approved and may be confiscated by the CBP.
Please understand that the CBP may inspect you and your belongings (luggage, vehicle and person) without a warrant.
Click here for the Customs and Border Control (CBP) complete list of prohibited and restricted items.
If you have a personal belonging such as a laptop, watch and camera which were made abroad, it might be supposed you had purchased them while traveling and should therefore pay duty on them. To avoid this confusion, you can register them with the CBP before departure (CBP Form 4457) or bring the sales receipt with you.
U.S. Department of Homeland Security Customs and Border Protection page for U.S. Citizens/ Lawful Permanent Residents