Legal Question in Real Estate Law in California

Assuming it's still a mobile home and taxed by the DMV rather than the county assessor, I'd make an appointment with the DMV to discuss your problem, which is probably not unique. One possibility is that they may allow you to surrender ownership, and hence tax liability, without substituting anyone as a new owner. In any case, I think you need to deal directly with the taxing bureaucracy, rather than apply pressure to the buyer, which likely won't work and (in the case of a lawsuit) would be expensive and time-consuming.