Consumers should expect higher prices on imported cars if President Donald Trump’s proposed 25% tariff on foreign-built vehicles shipped into the U.S. is enacted, according to car sellers.

Car makers said they plan to pass on the added costs to customers, which dealers and car sellers said could lead to a decline in sales. About 44% of all U.S.-sold cars were imported into the country last year. Because of the higher price tag, auto retailers said, they may curtail orders of vehicles built abroad and focus on used cars with higher margins.

The tariff could also, to a lesser extent, inflate prices for cars built in the U.S., because many of them use foreign-built car parts, car companies said.

“There is no doubt the auto retail industry will be adversely affected,” car dealer David Rosenberg said in an interview. “We’ll see price increases across the board and a lot of that will be passed on to the consumer. Sales will go down.”

Mr. Rosenberg, who has dealerships in New England, is taking Mr. Trump’s threat seriously. He has already begun stockpiling foreign-built Mercedes-Benz and Audi sport-utility vehicles based on worries that the tariff, if enacted, would make those vehicles more expensive to order from overseas.