Let’s start by explaining the differences between the parts.
OEM, Original equipment manufacturer part is a part made by the manufacture or made for them to their specification but an external company. A genuine part is a part supplied by the vehicle manufacturer in their packaging. Aftermarket parts are parts produced by any other company. Often they are reverse engineered to be a very close to the original specification. Many options are available which can vary wildly in price and quality. 
Toyota genuine Ignition CoilCopy Ignition Coil
A number of vehicle parts can be purchased directly from the OEM. The will be supplied in plain packaging and are exactly the same specification as the genuine part. Often these parts are better priced than a genuine part.
Genuine parts are available directly from the manufacturer and in some cases as a parallel import.
Specialized aftermarket suppliers are able to produce more durable parts than OEM, others just produce parts designed for direct replacement. 
When vehicle modifications are made often the best parts to use are the upgraded specialist’s parts. A company like this will reverse engineer the part and work out any weakness that may be present.  Manufacturers will design taking into account cost, noise, durability and performance. Aftermarket design may be aimed directly at performance and not be as concerned with noise or vibration.  A good example of this would be an aftermarket clutch. It will be able to handle more power than an OEM. As a side effect is it may vibrate or be noisier than OEM. Many other parts like ignition coils, Suspension arms, coil overs, brake pads and rotors all fit this category.
Carbon multi plate clutch
Items that are really cheap would have be designed for price without other aspects like durability considered.
In the aftermarket you have many options to choose from. If you are not familiar with the products you could get a low quality part just with clever advertising, instead of a well-engineered product worthy of the dollars spent.

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