Both you and the auctions company have ethics of some kind. For individuals, ethics come from religious beliefs, from community standards, and from their moral compass. Many businesses now state their corporate values (which often include ethics).
In this problem, a client is testing your ethics and the auction company’s ethical business practices by requesting a fabrication of a description.
Now, note that “ignorance” of a law is no excuse, but “negligence” is worse because the law is known and no effort is made to comply with it. Ethics are different from laws (usually), but many laws are based on ethics and have severe penalties/remedies.
Ignorance is: you don’t know about April Fool’s pranks, have never read a tabloid magazine, have never heard of a malpractice suit, don’t know what an elixir is, believe all the claims made by politicians, don't know what identity theft is … are not aware of the water quality in Flint, MI or heard of Erin Brokavich.
Negligence (“A failure to exercise the care that a reasonably prudent person would exercise in like circumstances”) is: Toyota delaying a recall for eight years, medical malpractice, failing to check references when hiring, …
Unethical (“not morally correct”) and illegal (“contrary to or forbidden by law, especially criminal law” is: falsifying hours worked, perjury under oath, broken international treaties, …, deliberately mis-describing an auction item.
Now, you should be aware of the Ethics Policies of the auction company and have some sense of the liability that this item description might have (both to the company and to you). Act intelligently and responsibly – that means that both the client and the auction company are informed (but, maybe, not too happy) about the event. Be aware of both the impact on the customer and the rights of the customer to seek a legal remedy.
Now, want to be a “used car salesman?” [note: Washington, DC-area government contractors are called "beltway bandits" for good reasons]