One of the largest problems faced by foreign traders in Guangzhou was finding a reliable medium of exchange that would enable sustainable trade with the Chinese. The Chinese were always willing to accept bullion, in exchange for tea and other products. This was because the Chinese were fairly self-sufficient and did not have a large desire for foreign goods. Specie was very expensive and difficult to acquire considering that the supply coming from South America fluctuated and it required a lot of goods to attain through a trade. Unable to afford to sustain high-level trading in specie, British merchants turned to the lucrative drug trade, obtaining trading rights for opium from India and importing it to the Qing Empire. Beginning in 1767 and rapidly expanding through the early 1800s, opium was illegally traded for specie with the Chinese and then reinvested in tea for importation to Great Britain.