If you sell it for the amount owed, it's little trouble. Just send in the payoff amount and a letter requesting release of lien status and the title. Once the financial intitution has finished it's paperwork, you'll get title in the mail (you can request it be sent express, but you'll have to pay for shipping). Then just sign it over to the new owner.
If you sell it for LESS than you owe, you'll have to pay off the difference. The buyer doesn't want a car with a lien on it. It doesn't mean you have to pay it off out of pocket; you could use another form of credit (credit card, personal loan from the same bank, home equity loan, etc.).
The bottom line is you have to pay off the loan to get the title, and you have to have the title to complete the sale.