Buying a car from someone who bought it with a riba-based loan

We ask Allaah to reward you with good for your keenness to
seek out halaal transactions and your caution against dealing in haraam,
especially your question about the issue you mentioned. Undoubtedly
borrowing with interest is the essence of riba (usury) that Allaah and His
Messenger have forbidden. One of the principles of fiqh on which there is
consensus among the scholars is: “Every loan that brings a benefit is riba.”
The lender in this case has committed a major sin, namely dealing in riba,
but the fact that he has committed this sin does not mean that it is haraam
to deal with him in buying and selling. The Prophet (peace and
blessings of Allaah be upon him) used to deal with the Jews, and they are a
people whom Allaah has described in His Book as dealing in riba and
consuming wealth unlawfully. 

Based on this, it is permissible to deal with a person who
deals in riba, buying and selling. 

There is nothing wrong with you buying a car from this
company, and there is no sin on you for that, in sha Allaah. 

And Allaah knows best.