N.Y. Comp. Codes R. & Regs. tit. 15, § 78.21 – Dealer plates.

(a) Dealer plates may be used for all purposes for which transporter plates may be used and, in addition, they may be used for the operation of any vehicle owned or controlled by the dealer and held for sale or demonstration, except:
1. (1) a vehicle maintained primarily for towing or for transporting property;
2. (2) a vehicle rented to another; or
3. (3) a vehicle used to transport passengers for hire.
(b) It is unlawful, among other things, to use dealer plates on vehicles owned by members of the dealer's family or employees, and on service cars.
1. (1) A dealer's plate may be loaned to a purchaser for a period of five days. The dealer will enter the number of the dealer's plate loaned in the block provided on the MV-50 (certificate of sale).
2. (2) Loan of a dealer or transporter plate to a purchaser or lessee of a vehicle which is to be registered in a state other than New York State. A dealer or transporter plate may be loaned to a purchaser or lessee for a period of five days for use on a vehicle which is to be registered in a jurisdiction other than New York State. The loan of the dealer and transporter plate will be recorded on the MV-50 in the block provided. The registration requirements do not apply to these loans.
3. (3) No dealer or transporter shall lend dealer or transporter plates to the purchaser or lessee of a motor vehicle, pursuant to the provisions of section 416 of the Vehicle and Traffic Law and these regulations, where the vehicle is to be registered in this State, unless the purchaser or lessee first exhibits to the dealer or transporter an insurance I.D. card as prescribed by the regulations of the commissioner or is exempt from insurance identification card requirements.
  (d) Loan of a dealer plate and vehicle.
4. (1) A registered dealer may loan his dealer plate with a vehicle owned or controlled by him for a period of not more than five days. A renewal of the loan covering the same vehicle and a dealer plate to the same person or firm is prohibited.
5. (2) [Reserved]
6. (3) The use of a vehicle and plate by a prospective purchaser for demonstration purposes for a period not to exceed 24 hours shall not be regarded as a loan of a dealer's vehicle and plate for the purpose of the regulation.
(c) Loan of a dealer plate to a purchaser.
(e) Motorcycle dealer plates are issued to a person, firm, association or corporation engaged in the business of buying and selling motorcycles. These plates are also issued to motorcycle transporters.
(f) Dealer plates on commercial vehicles.
A dealer may lend his dealer plates and truck, trailer or tractor to a purchaser or prospective purchaser in accordance with subdivisions (c) and (d) of this section. The purchaser or prospective purchaser may transport cargo and may engage in point-to-point operation for profit using the dealer plates.
(g) No dealer shall transfer, lend or permit the use of its dealer plates by another dealer or any other person except as specifically permitted by this section.
(h) No dealer shall use or permit the use of its dealer plates if the dealer registration is revoked, suspended or expired.
(i) As of January 1, 1994, all dealer and transporter plates shall expire on the same day as the dealer or transporter registration to which they are issued. If necessary, the commissioner shall prorate any fees for dealer or transporter plates issued for less than two years.
(j) If a dealer has not sold at least five vehicles during the previous 12 months of operation, then the commissioner may, upon written demand require such dealer to return one or more such dealer and/or transporter plates to the commissioner or his authorized representative within 10 days of such demand. It shall be a violation of this subdivision for a dealer to fail to surrender such dealer or transporter plates to the commissioner or his authorized representative within the 10-day period
(k) A set of dealer or transporter plates shall consist of one plate, starting July 1, 1993. Between that date and July 31, 1993, a dealer or transporter must mail to the department one plate from each two-plate set in the dealer's or transporter's possession. The department will not renew any dealer or transporter plate registration with two plates outstanding on and after August 1, 1993.