14 CFR 47.69: Limitations.

A Dealer's Aircraft Registration Certificate is valid only in connection with use of aircraft—

(a) By the owner of the aircraft to whom it was issued, his agent or employee, or a prospective buyer, and in the case of a dealer other than a manufacturer, only after he has complied with § 47.67;

(b) Within the United States, except when used to deliver to a foreign purchaser an aircraft displaying a temporary registration number and carrying an airworthiness certificate on which that number is written;

(c) While a certificate is carried within the aircraft; and

(d) On a flight that is—

(1) For required flight testing of aircraft; or

(2) Necessary for, or incident to, sale of the aircraft.

However, a prospective buyer may operate an aircraft for demonstration purposes only while he is under the direct supervision of the holder of the Dealer's Aircraft Registration Certificate or his agent.