Global shoppers looking to adorn their homes with found treasures such as Victorian crystoleums and Barbolla mirrors stroll London’s Portobello Road.

Across the channel, the Biennale des Antiquaires (whose Web site is bdafrance.eu) comes to the Grand Palais in Paris September 11-21. Artworks dating from the Middle Ages, prized manuscripts, and modern designs by Prouvé, Le Corbusier, and Ettore Sottsass are yours for the taking. But the hard work isn’t just the acquisition of, say, a fabulous one-of-a-kind hand-sewn Persian carpet. You’ve got to get it home and preferably in one piece.

Shipping or carrying big purchases back to America can be complicated. The cost often outprices the purchase. For most connoisseurs and amateurs, however, a really good find is priceless.

Whether shipping or carrying back items, it’s smart to familiarize yourself with U.S. customs law, and the laws of the country you are visiting. Consult the customs Web site’s “Know Before You Go” (travel.state.gov) document. A perfect end to an incredible trip does not include exotic smuggling charges and a prison sentence.

Consider exactly what you are buying and the value of it monetarily and sentimentally. “You’re allowed to bring up to $800 worth of merchandise tax-free, per person, back into the country,” an international fine art transfer broker, Racine Berkow, said. “You must declare everything, even if you bought it duty-free.”

One woman Ms. Berkow knows shipped a container of furniture to America without declaring anything, including a tiger-skin rug. “That’s an endangered species. She jeopardized her entire shipment,” Ms. Berkow said.

Passing through customs may seem casual, but agents aren’t lax on the law. “There are highly skilled teams of experts at Kennedy and Newark called the Art and Antiques Team,” Ms. Berkow said. “These people are sharp and recognize anything questionable.”

There is no finders/keepers logic with purchases of significant value to a country. “If you accidentally buy a Rembrandt from a dealer, chances are the dealer didn’t realize what he had, and chances are it was stolen,” Ms. Berkow said. “That’s patrimony of the country, and they will seize it at customs.”

Ask the dealer to ship the items for you. “Most reputable ones do,” the public affairs liaison for Customs and Border Protection, Lucille Cirillo, said. “Most include insurance with the shipping too.”

In developing nations, relying on the local postal service to send that 19th-century Gothic headboard is cheaper, but not the most secure method. Another advantage to dealing with someone with art shipping experience is that experts are conscious of details such as crating and climate control. It’s never a bad idea to shop for a good price, but the protection of your purchase is the priority.

International Convention of Exhibition and Fine Art (ICEFAT) is an organization of global fine art and antique transport companies that will assist with packaging and crating door-to-door. When shipping a package for yourself, you are exempt $200 in duty taxes. If you’re sending a gift, the exemption is $100.

For commercial shipments that exceed $2,000, a formal entry is required and the traveler may need the assistance of a customs broker to import the merchandise, Ms. Cirillo said. Be prepared to show proof with an invoice or a receipt. Have your tax ID number handy because, in some states, you could end up paying a usage or sales tax.

In reality, customs is there to collect taxes. The regulations aren’t meant to stop shoppers. “We’re not looking to create cases,” Ms. Cirillo said. “We’re there to look for terrorism and weapons of mass destruction, people avoiding paying tax money to our government, and people trying to transport stolen goods.”