Which of the following would represent an illegal act concerning commodity sales?

Buying based on incorrect quantity representation represents an illegal act concerning commodity sales because it involves misrepresentation and deception. In this context, accurate information about the quantity of a product is critical for consumers to make informed purchasing decisions. Misrepresenting the quantity can lead to consumers receiving less product than they expected, which not only violates consumer trust but can also breach laws and regulations that require fair and transparent business practices. This can result in legal consequences for the seller and undermine the integrity of the marketplace.

On the other hand, consumer returns based on personal preference are generally permissible and part of normal business operations. Purchasing without viewing the product is not illegal; however, it may carry certain risks that consumers should be aware of. Negotiating a price below market value is a common practice in business transactions and not illegal as long as both parties agree to the terms willingly. Therefore, option focusing on incorrect quantity representation stands out as a clear violation of lawful commodity sales practices.