The buyer has to send his CA Tax Exempt Sellers License
You cancel his eBay order if he did buy the item there. Cancel this properly protecting our account from ebay. See cancelling orders section to do this properly. Do not have record on ebay messages that you are completing the transaction outside of ebay.
Make sure item is listed on the shop if not please add it and make sure his account is Tax Exempt only AFTER they have provided their valid tax exemption.