I'm a bit confused....how can you be a co-borrower but not on the loan? Or did you mean you're a co-owner on title, but not on the loan.
If your parents want to give up their rights to the property -- usually this can be accomplished via quit claim deed. But that doesn't relieve them from their obligations to pay off the loan to the bank as long as the loan is in their name.
If you say that the property may be sold for more than the balance of the loan, and if neither you nor your parents want to keep the property, then go ahead and sell it on the open market.
But if you want to keep it, get yourself preapproved for a loan, and buy out your parents, assuming that the new purchase price and loan are enough to payoff your parents' loan. That may be the simplest thing to do.