How much is 2 years of back tags for buying a car?
Going to vary depending on the value of the car and where you live. They should be able to tell you if they answer the phone. Maybe even ask by email. There will be penalties & interest.
Each Country, State, or Province has it's own rules. Where I live the is no back tagging. You just pay for current year.
Anytime a car has back tags, it usually means something is wrong with the car's engine or transmission. Get the seller to get the tags and smog up to date. If he declines, find another vehicle.
How should we know? We don't know where you live, therefore we can't know what the licensing fees and penalties are, much less what the DMV can tell you. Where I live the DMV can tell me if there are outstanding fees to pay.