Two wins, not one
Selling something you don't use gives you two separate wins, and the smaller one gets all the attention. The cash is the small win. The bigger one is that from next month you stop paying for it: the rego, the insurance, the storage, the servicing, and the loan repayment if there's finance on it. That money keeps arriving every month for the rest of the loan.
The three lines in the table show what happens depending on what you do with each win. Most households do the first line. The last line is usually worth several times as much.
Don't forget the loan repayment
People remember to count rego and insurance and forget that paying out the finance frees up the repayment too. On the example figures that one line is worth two and a half years off the mortgage.
If you owe more than it's worth
This is common on caravans, boats and cars bought new. You have to cover the shortfall before the buyer gets clear title. It feels like a loss. Compare it with what the thing costs to keep. A $3,000 shortfall on something costing $11,000 a year pays for itself in about three months.
Get the payout figure, not the balance
The balance in your app isn't what it costs to clear the loan. Ask the lender for a written payout figure with an expiry date. It includes interest to the day and any discharge fee.