14 CFR §21.9

1. (a)If a person knows, or should know, that a replacement or modification article is reasonably likely to be installed on a type-certificated product, the person may not produce that article unless it is—
   1. (1)Produced under a type certificate;
   2. (2)Produced under an FAA production approval;
   3. (3)A standard part (such as a nut or bolt) manufactured in compliance with a government or established industry specification;
   4. (4)A commercial part as defined in § 21.1 of this part;
   5. (5)Produced by an owner or operator for maintaining or altering that owner or operator's product;
   6. (6)Fabricated by an appropriately rated certificate holder with a quality system, and consumed in the repair or alteration of a product or article in accordance with part 43 of this chapter; or
   7. (7)Produced in any other manner approved by the FAA.
2. (b)Except as provided in paragraphs (a)(1) through (a)(2) of this section, a person who produces a replacement or modification article for sale may not represent that part as suitable for installation on a type-certificated product.
3. (c)Except as provided in paragraphs (a)(1) through (a)(2) of this section, a person may not sell or represent an article as suitable for installation on an aircraft type-certificated under §§ 21.25(a)(2) or 21.27 unless that article—
   1. (1)Was declared surplus by the U.S. Armed Forces, and
   2. (2)Was intended for use on that aircraft model by the U.S. Armed Forces.