You can't just buy shares from a registered securities professional (e.g. broker, dealer or underwriter) and then start engaging in "off the books" securities transactions. That is, unless you don't mind a lengthy prison sentence.

The activities you are describing (buy securities and then sell "stakes" and/or profits interests and/or voting rights) fall very squarely within many of the various definitions of securities professional required to register and comply with these extensive regulations. For example, see definitions 11 and 12 here [1]. Also note that the definition of "sell" includes selling any interest in a security--not just selling an entire security outright.

> (11) The term “underwriter” means any person who has purchased from an issuer with a view to, or offers or sells for an issuer in connection with, the distribution of any security, or participates or has a direct or indirect participation in any such undertaking, or participates or has a participation in the direct or indirect underwriting of any such undertaking; but such term shall not include a person whose interest is limited to a commission from an underwriter or dealer not in excess of the usual and customary distributors’ or sellers’ commission. As used in this paragraph the term “issuer” shall include, in addition to an issuer, any person directly or indirectly controlling or controlled by the issuer, or any person under direct or indirect common control with the issuer.

> (12) The term “dealer” means any person who engages either for all or part of his time, directly or indirectly, as agent, broker, or principal, in the business of offering, buying, selling, or otherwise dealing or trading in securities issued by another person.

Here's just a random example from Google of the penalties your hypothetical "off the books" broker could face [2].

[1] https://www.law.cornell.edu/uscode/text/15/77b

[2] http://companycounsel.net/2010/01/21/you-dont-have-to-know-t...