A dealership lot in Miami Lakes, Fla., for used-car retailer CarMax, which says high prices, inflation and rising interest rates are slowing demand. Auto executives for months have expressed confidence there will be eager buyers for all the vehicles they can build . A worsening economic picture is putting that theory to the test. The U.S. auto industry is expected to report flat new-car sales for the third-quarter, despite earlier predictions that this year’s depressed selling pace would accelerate in the second half, analysts predict. Most auto makers are scheduled to report third-quarter sales results Monday.