That’s a great question, @Mark-Brink!
Marking a customer as inactive in QuickBooks Online does not automatically change or remove that customer’s information in Mailchimp. The integration doesn’t pass status updates like “inactive” or “deleted” from QuickBooks to your Mailchimp audience.
If you want to unsubscribe a contact in Mailchimp, you’ll need to do that directly within Mailchimp.
So in your case — when a property sells and you inactivate the current customer in QuickBooks — you’ll also want to manually manage that corresponding contact in Mailchimp to make sure your audience list stays up to date. I hope you found this helpful. Feel free to reach back out in the Community if you have additional questions.