Katt Williams Sued -- Accused of Stiffing Used Car Seller

Which is more unlikely ... that self-proclaimed multi-millionaire comedian Katt Williams recently purchased a 10-year-old used SUV ... or that he allegedly failed to pay for it?

According to a lawsuit filed last week in L.A. County Superior Court, Katt bought a 2002 Ford Expedition on October 30, 2012 from a guy named Khosrow Gharib.

Mr. Gharib claims Williams got the wheels, but never paid and now owes him $7,500, which he says is the Kelley Blue Book value of the car. Gharib says he has tried to get Williams to pay up, but so far ... no luck.

We also reached out to Katt -- for a comment -- but no word back.