Thank you for using our forum. My name is ***** ***** I hope to assist you today.
I am sorry to learn about this situation.
Unfortunately, there is no guarantee for tenants to remain in their rental unit any longer than the stated lease agreement. So if your lease agreement gives you until October of this year, and your landlord has sent you a notice of non-renewal, there is no legal requirement for the landlord to extend your tenancy.
(As an aside, I would strongly encourage you to refrain from entering into any type of agreement such as the one proposed by your landlord - they are suggesting that you assist them in committing fraud (they are trying to skirt CA property tax and sales tax laws), while this does not directly implicate you - you do not want anything to do with this).
Also, any type of "land sales contract" (sale by owner or lease to own agreement), must be in writing to be enforceable, you should also have an attorney (your attorney, not the sellers) review these contracts, they can be useful to help new owners buy a home, but in my experience they are fraught with pitfalls for most parties and a badly written contract can cause even more problems.