Legal Question in Real Estate Law in California

Yes, you would need a license to bid on behalf of others if you expected to earn a commission, and unless you are supervised by a broker, you would need a broker's license, not just a salesperson's license.

Any party can bid at a trustee's sale, but it is considered bad form for the trustee to bid on his own behalf and probably worse if he bid for an undisclosed client. See Copsey v. Sacramento Bank (1901) 133 Cal. 659. Therefore, I would advise not being a trustee and a bidder at the same time.

Also please note that so-called "trustees" doing foreclosure sales are not true trustees and have quite different duties than trustees of trusts, but nevertheless bear certain risks for either failure to disclose known defects in the property sold, or for conducting an improper sale or doing so with defective authority. You will be dealing with large sums of money and need to understand the business very thoroughly or you'll be risking your clients' and your own money in every deal.