買由公司名義持有的住宅樓宇問題

If the owner is a limited company, techincally speaking, there is a minutes of meeting of directors authorising a director to sign the legal documents e.g. sales and purchase agreement ( S & P ), assignment deed etc. on behalf of the company and with the common seal, not simply the rubber chop, being affixed on the documents. You can or ask an agent to carry out a company search to know the details of the company, directors and shareholders etc. in the Companies Registry.
If you are signing the provisional S & P likely in the office of the property agent, the document should be signed by a director with the company's chop stamped on it to state out that the person is signing the document and with that authority on behalf of the limited company.
A formal S & P will then be executed before lawyers within a reasonable time as stated in the provisinal S & P.