You can't do this in a query on your Dealers BO. You could do this in a Customer Presentation of that BO by showing a totals band only & not showing the detail band.
If you want to do it in a query, you will have to have another BO, say DealerTotals that you build via rules / process.
daleb,
Take a peek at the Order management application in the sample folder from Aware, this comes with your installation. This should help you understand using the objects as Tom mentioned to achieve your goal.