I am not a tax advisor and this should not be taken as advice. This is simply a summary of what I was told to do by the CDTFA customer service line as someone who sold stuff on Amazon and Ebay in addition to my other sales. They are very helpful and you should give them a call with your questions.
I filed my total sales including ebay and amazon, then on the next page, there are prompts to note how much of the sales are considered non-taxable (because you either sold them to another retailer, you sold them out of state, etc) There is an option called other, and under that there is an option to select ‘marketplace’. I put my amazon and ebay sales added up together under marketplace, and then made a note that it was from Amazon and Ebay combined (you can’t file seperate marketplace lines). That part of my sales was not included when calculating the total sales tax I owed.
Definitely give CDTFA a call because they are there to help you.
Not a boutique owner but some ideas -