Well, the good news is that an insured buyer won't have to worry. If you have an owner title policy, the title insurance company will cover the lien, then decide if they want to pursue the seller.
Yes, the seller IS ultimately responsible and they SHOULD pay for it. If the matter goes to a court, I am certain they would be REQUIRED to pay for it. The title company will decide how much money is involved and how much trouble it will be to force the issue with the seller.