The car they arrive in is part of the price of the car they leave in
A trade-in is appraised and valued on the enquiry, and the net is derived rather than recalculated by hand every time somebody asks.
Their car is free text, on purpose
The vehicle being traded is described rather than picked from a catalogue. A foreign key here would fill your own product catalogue with your competitors' models, one appraisal at a time.

It asserts nothing about the sale
A trade-in is the one sub-process that raises no outcome at all. Bringing a car to be valued is not evidence of a decision — plenty of people are just finding out what theirs is worth.

Worth knowing The decisions behind it.
- 01
Appraised value and agreed value are separate. The gap between them is a negotiation, and flattening it into one number loses the negotiation.
- 02
It attaches to the enquiry rather than to a particular vehicle interest, because people trade one car against whichever of three they end up choosing.
The rest of the sale.
See it on your own showroom floor
Set up your dealership in a few minutes, put a real enquiry through it, and judge it on that.