If an investor bought a stock, could another private party access that information anywhere? Does the SEC/exchange itself create a real time/ historical record of who holds what stocks, and is that available to the public?
I do not believe that the exchange is capable of tracking down the person or legal entity who has been part of any recorded transaction and in particular linking the activity of market intermediaries to the ultimate interests of beneficial owners. However, under certain conditions, the person or legal entity has to report to the SEC his identity and his stake in the company. The concept of beneficial ownership governs disclosure obligations.
Insiders and 10%+ beneficial owners
Changes in ownership for insiders and 10%+ shareholders must be announced with "form-4".
"Form-5" is an SEC filing submitted to the Securities and Exchange Commission on an annual basis by company officers, directors, or beneficial (10%) owners, which summarizes their insider trading activities.
5%+ beneficial owners:
Filing "SCD 13-D" is required by 5%+ owners within 10 days of the acquisition event.
Filing "SCH 13-G" is an annual filing required by all reporting persons owning 5% or more of the company.
There is a great answer as to when an investor has to file. The only other piece of information that you might have available to you, is the broker number of the buyer or seller as listed on the trade notification.
For most of the big firms this won't help you as their retail , buy side and their own internal flow will all be seen under that number.
It can help you if the buyer/seller is a fund with their own broker number.