If you want to sell your car while it is still being paid off, you need to contact the lender and ask what their policy is. Often, lenders allow the buyer to pay the remainder of the loan in exchange for a free and clear title, but policies vary, so it’s important to confirm with your lender.
Once the loan has been paid off, you may see a slight decrease in your credit score, but it is usually temporary as a result of closing an active credit account. If you have more specific questions, your lender should be able to help.