If you got the credit in 2008, and sold the home (or otherwise stopped using it as your main home) in 2010, you were supposed to report the event on your 2010 return. You owed immediate repayment of the credit balance, or a partial repayment, or no repayment, depending on the financial circumstances of the sale.
If you have been paying an extra $500 every year since, that was an error. You never owed that money, because you should have cleared off the credit one way or the other in 2010.
At this point, you have to lie and say you live in the home, and keep repaying the $500 per year until you close out the $7500; or you will have to file an amended 2010 return to close out the credit properly. You can claim the annual payments that you have been making incorrectly, toward the amount you owe. But you will need to do this manually or see a professional, since Turbotax does not go that far back.
