ask for a copy of her resale certificate. you have the right to do that. MOST sellers who charge sales tax dont have them, and cant rightfully charge it. I have asked sellers for it before and all of a sudden the tax was waived.
There are two things in CA - the resellers certificate and the sales tax license. You use the same number for both.
The reseller certificate is a form where they certify that the good they are buying from you is for resale. It also lists the number.
The resale license is something the state sends businesses with the number on it.
Many business owners have even have trouble with the difference...
Here's a link to the CA Board of Equalization's sales tax number verification page. You need them to provide you with number. You can use this link to verify it's a legit number:
https://efile.boe.ca.gov/boewebservices/verification.jsp?action=SALES
Many people actually fake numbers and say they are reselling the merchandie to avoid paying tax.
Technically in most cases, you are supposed to charge sales tax for the county/city you are delivering to - not the one you're in unless the customer picked it up from a physical locations like a retail store. Most people don't bother and post all their sales to the county they sell from and don't figure it out until an audit.
Let me know if you have any specific questions and I'll try to help. I used to be an accountant in CA and worked with sales and use tax quite a bit.