How do I filter out DEALER ads and find cars for sale by PRIVATE OWNERS??

Under the "Filter Results" section on the bottom left of the page, you will need to select "Private Seller" and the dealer ads will be deleted.

this option does not exist

Yes it does. Look under Seller Type.

I don't see the option either.

Used to be there. They seem to have recently removed private sellers?

The option shows but is muted and does not allow you to choose it. Why? How can you make it an active choice? I have occasionally been able to choose it, but not most of the time.