You need to talk to a Wisconsin attorney before you do anything. What you propose to do would be a crime in Texas (hindering a secured creditor, Tex. Pen. Code 32.33). I suspect there might be a similar statute in Wisconsin. Even if there is no corresponding criminal statute, you would certainly be facing a conversion suit by the creditor. It sounds like you have a claim against the seller. You will have to determine whether it is worth the expense and effort to pursue him if you can find him and if he has any assets upon which to collect.
I am providing this general reaction to your question for discussion and informational purposes only. My answer could change depending on the specific facts of your situation. This answer is not intended to be legal advice and providing this answer or your reading it does not create an attorney-client relationship between you and me. I create such relationships only when there is a signed attorney fee contract between my client and me.
Because the lien was not satisfied, you took ownership of the vehicle subject to the lien. This means that the bank may repossess it even though you own it. If you sell the vehicle, you may be prosecuted with a crime or sued by the bank, as the other contributor wrote.
You may be able to negotiate a release of the lien directly with the bank. The bank may release the lien with payment of some money. Banks usually don't want to repossess vehicles. They want money, and the bank may accept a lesser sum to release the lien. You may find it distasteful to pay more money, but it may be better than losing the vehcile.