Never gone through what you are going through, but I bought a used Jeep last year, was a voluntary surrender. Long story but I ended up talking with and meeting with the previous owner to get some parts, and a key. He was happy I bought it as it finally closed a bad chapter of his life and he could finalize things with his bank. He told me the biggest thing was he was really trying to work with his bank and not fight or deny things.
So my suggestion is to call them, ask them about things and let them know you are aware it sold. Even if you can't start paying it back now at least you know and they may work with you and not chase you down. In the Jeep guys situation they wrote off a fair amount of the amount he owed them he told me, but he still had to pay some back, but he was thankful they worked with him. Nice guy, I still feel guilty sometimes, then I drive and that goes away. :)