I sold a car in 1999,(CA.) but have lost all records of sale. The
state tax franchise board has now suddenly garnished my paycheck for a
reg fee ($250) that was not paid in 2000. what do I do? I can't prove
I sold the car and need to limit my liability

This is pretty simple, if you ask me. You need to submit a release of
liability. Sure, this sale happened a long time ago, but maybe if you
remember approximately the date of the transaction, you can write it
up for that date. If that is not accepted, at least write it up with
today's date and limit your liability starting today!

Go to http://www.dmv.ca.gov/forms/reg/reg138.htm

If they are being unfair, file a complaint!
http://www.dmv.ca.gov/forms/inv/inv172a.htm