The draconian tax law of Hong Kong requires any person who carries on business in Hong Kong to register with the Inland Revenue Department. This registration costs a few thousand dollars and is evidenced by a business registration certificate (BR).
Most businesses have no problems with this because if they do set up a company and trade in Hong Kong, the application process automatically involves obtaining a BR.
But what if you are an e-merchant in Lancaster, UK, who sells predominantly to Hong Kong residents residing in Hong Kong? Technically, you may be doing business in Hong Kong, and the source of your profits will also highly likely be regarded as Hong Kong. You can check the FAQ and departmental practice notes yourself. That means that you are obliged, under the HK tax law, to register a BR for your business and be liable to Hong Kong tax.
Needless to say, if you live in HK and: are a private tutor, sell things over taobao to other HK people, or do any other business, you may be liable too.
There are certain exemptions for small business with less than $30,000 turnover involving HK, but you still need to actively apply for a certificate of exemption.
If you have concerns about this, you can get an advance ruling from the tax authorities by sending them some forms, and an appropriate cash bribe: http://www.ird.gov.hk/eng/pdf/e_dipn31.pdf