How to return a true or false by comparing two separate objects by unique email addresses

Hi,

I have three objects,

1.Buyers

2.Sellers

3. Appointments

When an Appointment consist of an Appointment and a Seller IF there is a Buyer record with the same email address when I edit the Buyer record I want to show a notification saying this client is also a Seller.

I have experimented for a couple of days now with different connections and attempts to do this, but the route problem seems to be that without the Appointment being made up of all three objects there is no way to trigger the notification.

Is this a case of needing the Javascript lookup function, or is there a way within knack please?

Kind regards

Paul