If the owner that sold the X5 to the party you are buying the X5 from signed the title over and your seller never signed the title as the purchaser, as far as anyone knows the X5 was never sold to him. Sales tax is paid at the time title transfer. Sounds like your party has never signed the title or licensed the X5 in his name, that his only intention was to flip it for a profit without paying sales tax. Ask the DMV what to do if you still want it.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas