“Broker” means every person, other than a stock agent, who in this state engages either wholly or in part in the business of (a) dealing in any security issued by others, (b) underwriting any issue of such securities, (c) purchasing such securities with the purpose of reselling them, or (d) offering such securities for sale to the public. Authority to act as a broker shall not be implied from an appointment executed by an insurer appointing a stock agent of that insurer.