As a starting point, Arizona does not recognize common law marriage.  So no matter how long you cohabitated, you do not have any legal right to community property with a partner.  However, since both your names are on the title to the car, you jointly own the car.  If you want to sell it, both of you have to sign.  If one of you wants to buy the other out and you can agree on an amount, then you two can sign over title to the buyer.  If he feels he has a greater ownership stake than you because he paid more and he wants to therefore reap a greater share of sale proceeds, he would need to go to court to get a judge to decide what percentage of the car belonged to each of you.  That would probably cost more to do than it would be worth.