I wouldn't pay it and I wouldn't phone them again. I would send them a letter, certified, return receipt requested. The letter would state what I paid for the car, That it was an arm's length transaction at fair market value, that I paid the appropriate tax, And that there is nothing further to discuss. I would inform them that if they disagree, that the burden of proof is on them and to submit their evidence to you for consideration, and that all correspondence must be by mail. Doubtful you would ever hear back. I have been in business for 40 years, and have been constantly harassed by petty, small minded, power hungry bureaucrats. Local, state, county, federal, IRS, EPA, Dept of Labor and industry, etc. One thing I have learned is to never, ever, communicate verbally or via email. Always and only via certified mail. It diffuses their ability to enjoy the power trip, raises concern that they could be held accountable for mistakes, and all but the most hard-core will close the file and move on to an easier mark.