Spare parts do not always comply with laws and regulations regarding quality and performance. For example, after-sale headlights may not meet local regulations in your country, or replacement engine parts may not meet emission standards in your country.
It is important to determine the correct details and characteristics of the part before buying. Depending on factors such as quality and brand name, a replacement part can cost less than or the same as a new OEM part.
Quality transmission parts made by companies that specialize in a specific area are considered performance-enhancing auto parts and usually cost more than regular new OEM parts. If you're not ready to spend money on personalizing your car, you don't have to go this route.
OEM auto parts used for vehicles are made by the same manufacturers that make the cars. The only factor is that these parts are not new. They have been used before, possibly in other vehicles of the same brand, and have somehow found their way to a junkyard or rescue station.
In this case, before buying used auto parts, it is important to consider that the parts used by a particular ABC vehicle will most likely only fit ABC vehicles of the same model and year by company standards. This is because every year the manufacturer can make minor changes or improvements to the design and specifications of auto parts for each particular model.
Since all OEM auto parts from a particular automaker are of the same quality and have been tested by the manufacturer for optimal performance, you shouldn't compromise on this front. Remember that this is not the case with spare parts.