Any suggestions how to resolve this?
I appreciate you for getting back to this thread, @The Old Oracle.
I recognize how relevant tag both Asset Purchase and Part-Exchange to your business is. Currently, this option is unavailable in QuickBooks Online as only one tag per group can be used in a single transaction.
Rest assured our product engineers are continuously working to roll out enhancements based on our customer's requests. That being said, I suggest sending a recommendation to our product developers. They'll review this idea and might consider updating it in the future.
Here's how:
- Go to the Gear located at the top.
- Under Profile, click Feedback.
- Enter your recommendation and select Submit.

You can also utilize this link to check the status and keep you in the loop on our progress of your request: Customer Feedback for QuickBooks Online.
In the meantime, you can enter the new asset as a Bill, with the part-exchanged vehicle as a Credit Note. This way, you can tag both vehicles.
I also encourage you to visit this resource that will guide review your transactions and put them in the correct accounts: Categorise and match transactions.
As always, add a comment below if you have any other questions about managing transactions in QuickBooks. I'm here to help always. Take care!