Dealer. Any person engaged in the business of selling firearms at wholesale or retail; any person engaged in the business of repairing firearms or of making or fitting special barrels, stocks, or trigger mechanisms to firearms; or any person who is a pawnbroker. The term shall include any person who engages in such business or occupation on a part-time basis.
27 CFR § 478.11
When used in this part and in forms prescribed under this part, where not otherwise distinctly expressed or manifestly incompatible with the intent thereof, terms shall have the meanings ascribed in this section. Words in the plural form shall include the singular, and vice versa, and words importing the masculine gender shall include the feminine. The terms includes and including do not exclude other things not enumerated which are in the same general class or are otherwise within the scope thereof.