Section 4. Every vendor engaged in business in the commonwealth and making sales of tangible personal property or services for storage, use or other consumption in the commonwealth not exempted under this chapter, shall at the time of making the sales, or, if the storage, use or other consumption of the tangible personal property or services is not then taxable hereunder, at the time the storage, use or other consumption becomes taxable, collect the tax from the purchaser and give the purchaser a receipt therefor in the manner and form prescribed by the commissioner. The tax required to be collected by the vendor shall constitute a debt owed by the vendor to the commonwealth. Such vendor shall collect from the purchaser the full amount of the tax imposed by this chapter, or an amount equal as nearly as possible or practicable to the average equivalent thereof; and such tax shall be a debt from the purchaser to the vendor, when so added to the sales price, and shall be recoverable at law in the same manner as other debts.

Notwithstanding the provisions of this section, the excise imposed by section two upon the storage, use or other consumption of motor vehicles or trailers shall be paid by the purchaser to the registrar of motor vehicles in the manner prescribed by the commissioner. The vendor thereof shall not add the tax to the sales price and shall not collect the tax from the purchaser. The vendor thereof shall, however, furnish to the purchaser, the registrar and the commissioner a sworn statement of the sale upon a form prescribed by the commissioner, giving such information as the commissioner may require for the determination of such tax. For purposes of such determination, the sales price of any motor vehicle, except a motor vehicle purchased from a vendor registered under this chapter who is regularly engaged in the business of making sales at retail of such motor vehicles, shall be the actual amount paid by the purchaser to the vendor for the motor vehicle or the clean trade-in value of the motor vehicle, whichever is greater, regardless of the actual condition of the vehicle. ''Clean trade-in value'' for a motor vehicle shall mean the clean trade-in value or equivalent or successor values listed in the National Automobile Dealers Association used car guide or other value guides, whether published in print or electronically, or default values as determined jointly by the commissioner and registrar. The commissioner may establish rules and regulations for accepting values below these values based on the condition of the vehicle at the time of sale or other factors as may be appropriate.

For the purpose of this section, the term ''motor vehicle'' means any self-propelled vehicle designed for use and used primarily upon the highways.