Owner traded (personal) vehicle not listed on balance sheet to clear payable invoices from vendor. How best do you record this in QBO?

Glad to see you here in the Community, lrnanne.

Let me provide you with steps for paying your bills with traded vehicles in QuickBooks Online (QBO). 

To do this, consider marking as paid the outstanding bills and entering the amount of your traded vehicle. Here's how:

1. Go to +New, then click Pay Bills.
2. The outstanding Bills will show on the screen.
3. Mark checks the bills you want to pay with the traded vehicle.
4. Enter the amount in the Payment column.
5. Once done, hit Save.

I also recommend consulting your accountant for accurate steps in recording the paid bills and ensuring the accuracy of your books.

Just in case you want to reconcile an account in QBO, visit this article for guidance: Reconcile an account in QuickBooks Online.

Your follow-up questions are always welcome in the Community. We are here to assist you at any time.

@lrnanne 

Obviously, the response from @GenmarieM doesn't address your issue. 

Create a journal entry that increases the appropriate owner's equity account (depending on what business entity this is - sole proprietorship, S-Corp, etc.) and creates an A/P credit that can be applied to the vendor's bills.  The journal entry is a debit to A/P for that vendor and a credit to the appropriate owner's equity account for the fair market value (FMV) of the vehicle.  You should have some documentation to support the FMV (blue book, etc.).  After creating the journal entry, you now have the FMV of the vehicle available to apply to the vendor's A/P balance.