[1][2][3][4] Indeed, it is not necessary even for the contracting parties themselves to assign a monetary value to the goods for a part exchange deal to be held to be a sale.
If goods/services have obvious monetary values, then a part exchange deal can be held to be a sale.
Technically, the customer is making a "supply" for the discount given, providing the old car for an amount equal to the monetary discount, and the dealer is also making a "supply" for the full price, providing the new car for its full sale price.
[2][5] Car dealerships are one business sector where part exchange deals are common.
In the housing sector, for example, only a few businesses will make part exchange deals.
The major accounting considerations are making provisions at year's end for part exchange stock that remains unsold, and for the predicted marketing costs of selling it.