What was the chief European import that the Chinese were interested in?

Opium. The history of opium in China began with the use of opium for medicinal purposes during the 7th century. In the 17th century the practice of mixing opium with tobacco for smoking spread from Southeast Asia, creating a far greater demand.

Imports of opium into China stood at 200 chests annually in 1729, when the first anti-opium edict was promulgated. By the time Chinese authorities reissued the prohibition in starker terms in 1799, the figure had leaped; 4,500 chests were imported in the year 1800. The decade of the 1830s witnessed a rapid rise in opium trade, and by 1838, just before the First Opium War, it had climbed to 40,000 chests. The rise continued on after the Treaty of Nanking (1842) that concluded the war. By 1858 annual imports had risen to 70,000 chests (4,480 long tons (4,550 t)), approximately equivalent to global production of opium for the decade surrounding the year 2000.