Aftermarket

[Britney]

Parts made by an outside firm to replace [OEM] (Original Equipment Manufacturers) parts. Commonly used in automotive purposes, but the aftermarket exists in virtually every industry.
In other words, these are NOT Genuine <fill in name of manufacturer> parts. The quality of aftermarket parts can vary widely, some engineered to be better than [the originals], some poorly fitting and otherwise inferior crap. [Caveat Emptor] when buying aftermarket parts! But sometimes when the original parts are discontinued or the manufacturer has gone out of business, the aftermarket may be your only choice.

[Kai]

The market for spare parts, accessories, and components for motor vehicles.

[Britney]

the selling of new parts for cars that have already been manufactured: