There are 2 different Deals modules, one allows only 2 trades per deal and the other, unlimited trades.

When you click on the Add New Trades button, you are presented with a input screen to enter in the VIN, this can fill in all the specifics of each selected trade. You can also manually enter in the information. The [Stock Trade] button will add the selected unit to your inventory and tag it as stocked in. The Delete trade will remove the trade from your deal screen but not from your inventory if it has already been stocked in.

The update button will total and carry over the net figures to you Finance screen. You should click this button any time you manually update trades.