偽造畢業證書

The act of buying a fake college degree is not illegal. But once you have bought the fake degree and come into possession of it, it is then illegal.
According to Hong Kong Ordinance Cap. 200 <CRIMES ORDINANCE> SECT 75 - Offences of possessing a false instrument - Part 2:
A person who, without lawful authority or excuse, has in his custody or under his control an instrument which is, and which he knows or believes to be, false, commits an offence and is liable on conviction on indictment to imprisonment for 3 years.
Keeping it for "personal collection" is not an lawful excuse.