The difference between the two is in the manufacturing tolerances for components and measurements. The OEM part is built to the original factory specifications for that part. An aftermarket part does not necessarily meet those specifications and are just slapped together or use components that do not meet specifications. That being said, buying a non OEM spec part can be a crap shoot, sometimes you win and sometimes you loose. Not that you don't occasionally get a bad OEM part, but you odds are a lot better at getting something that lasts. The majority of aftermarket replacement parts made today are sourced in China and built by what would be considered ***** labor in this country, the reason they are so cheep. I could go off on this, but it is not the subject of the post.
In some cases, you can't get OEM parts for older vehicles and in that case you are stuck with lesser part. An example of that is some of the skins on your vehicle, IE. fenders and quarter panels. In that case you make do with what you get. There is a lot more to be said about this, but I wont belabor the point.
Hope that helps.
Good luck,
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