While most things are negotiable, there are a few items which by law are allocated to the seller or buyer. Even if allocated, so long as the legal obligation to perform is done, the parties can negotiate who pays for what. See this memo as to costs applicable in Los Angeles including retrofit. See this pdf: Seller pays for Retrofit and LA compliance certificate
As to allocation of title, escrow and some other closing costs, see this memo from a title company: Guide To California Closing Costs