Can one benficiary of a trust buy out another beneficiary? We live in california and that's where all trust property resides. My mother has offered

It is very common to do buyouts in trust and estate cases.  The assets are often valued by experts (appraisers) to make sure everybody gets treated fairly. However, the actual valuations are by agreement so you do not have to agree to the appraisers valuation. Typically a written "distribution agreement" is prepared to show what everybody is getting.