How did the mechanic come to be in possession of the vehicle? There does seem to be a point where the mechanic can say that you haven’t paid for the repairs performed so they can sell the vehicle for the cost of the repairs. (Same thing a dry cleaner can say if you don’t go back and pick up your cleaned clothes). However if he didn’t perform any repairs – don’t know that this would apply.
As far as the sheriff – if you swear out a warrant on the mechanic, his office is duty bound to serve the warrant. You can serve a warrant on anyone for almost any reason – theft is a valid reason, so he’s wrong in saying that its a civil/criminal matter. The courts decide that, not the sheriff, generally in terms of monetary value. As someone else said, he’s not going to go open an investigation because no one was shot/killed/physically brutalized. Theft of the automobile would be another matter for the sheriff to take immediate action on, but since you left the vehicle with the mechanic-I would pursue that avenue, just the value of the vehicle/gains from the sale of the vehicle.