The fact that the ad does not specify vehicle mileage would concern the hell out of me, "highway" or not. I can't imagine what the seller was thinking in NOT posting the mileage.
Highway miles simply denote that the owner alleges the car was not subject to a lot of stop and go city driving, which is more wear on engine components, clutch, brakes, etc. Go to kbb.com and load in the data for the vehicle shown. You can play with the mileage to see how many miles would calculate the $9000 price. THEN contact the owner if it seems fair.