Tax on used vehicles is paid when registering the title (actually it's there on new cars too, but the dealer includes it in the "tax, tags, and delivery" fee). I own a "parts car" which I've never titled, so I've never paid tax on it.
Carfax can only tell you if the car's been involved in something which an insurance company has reported on. It can't tell you the service record or anything like that.
The best thing you can have when buying a used car, is someone to take with you that knows a lot about cars.
