My purpose in writing this guide for the eBay community is to make as many as possible aware of the benefits and even dangers of choosing aftermarket vehicle parts in place of original equipment factory parts.

First, be wary of vocabulary vendors and sellers use to tag their products. 

Factory original or NOS (New Old Stock):  These are the best parts for your vehicle because they were manufactured under the specifications and processes of the automaker itself or one of its subcontracted manufacturers.  For example, you can purchase a factory original door from a GM collision repair facility or another set of Firestone ATX tires for your 1985 Ford Bronco.  The door was made for GM by GM itself, and the tires were made by Firestone for use on Ford trucks sold new at dealerships around the country.

OEM:  Many sellers advertise OEM (original equipment manufactured) parts, but all that means is that they fit LIKE original and SHOULD perform as good as original.  With common wear and tear parts like tires, spark plugs, brake pads, shocks, air filters and the like, there is usually never a problem with these aftermarket "OEM" styled items.  Why is that?  Because they will usually be replaced befre their service life has expired. BUT with highly specialized ignition components, such as engine control computers, and the neglected parts such as emission control products, engine sensors, and even carburetors, be wary of their roadworthiness.

Aftermarket:  This word has gained more appeal in America's eyes every year because it means putting your car back in shape without the expense of dealer parts.   Aftermarket can also be another way to spell DANGER when you deal with collision parts and other critical paraphernalia.  What you may save in dollars and cents may cost you more than you bargained for sooner than you think!

Now that I have cleared the basic vocabulary, you'll be able to read my advice carefully.  Its short and to the point, but don't let its brevity make you assume unimportance.

If you have never feared breaking down on the coldest winter day of the year, in that heavy rain, the sweltering 110 degree heat, or the beautiful spring afternoon, you must already be on top of your vehicle's maintenance just as you are on getting a shower each day.  Excuse me for those dirty folks out there!  If you aren't...read on.For strictly enhancing or customizing the look of your vehicle, there are usually little problems.

As long as you repair your vehicle on a comprehensive maintenance schedule, you should theoretically never break down.  This includes checking your tires daily, battery and charging system monthly (more during winter and summer), all fluid levels weekly, belts and hoses at every oil change, and fuel delivery/exhaust systems yearly.  That pesky check engine light actually means something too, so its not okay to drive around with that on either!

When parts do get replaced, it is always best to use factory original equipment as they tend to perform best through the service intervals required by your vehicle's manufacturer.  Aftermarket or OEM styled underhood components could have a shorter service life, leading to unexpected failure just when you are on your way to pick up your date for a night on the town.

If you have ever been in a collision, notice the way your car crumpled or absorbed the impact.  That is no accident, as years of research and development were invested in your vehicle's design.  The metallurgy of body panels such as doors, hoods, fenders, quarter panels, floor pans, structural steel, titanium, or aluminum strive to raise your chances of coming out alive.  Even aftermarket glass can be risky business. Notice how easy it is to dent an aftermarket fender, or bend an aftermarket bumper.  They mawr if you look at them the wrong way!  Factory original parts are a must...and especially watch out when the insurance companies are paying for damages.  Most states allow them to opt for generic aftermarket parts that just meet federal vehicle safety standards instead of exceeding them.  They want to pay the least possible.  You have the right to request original parts, from a fender down to a light lens, even if you must pay more out of pocket!  Avoid use of aftermarket collision parts unless you're vehicle will never be driven.

How about that new material you just installed over your seats?  Is it fire retardent?  Or that big sound system you just installed in our trunk...is it taxing your alternator and discharging your battery?  Better find out before its too late!

For strictly enhancing or customizing the look of your vehicle, there are usually little problems with aftermarket items such as grilles, running boards, and even headlight lenses these days, since factory plastic crystalline lenses tend to yellow and cloud as well.

Before I conclude, I'd like to point out another thing.  Unless you really like waiting 5,000 or 15,000 miles before changing your engine oil, you don't need to pour high mileage synth that costs upwards of $5 per quart into your engine.  Store brand  $1 10W-40 will suffice just fine for those 3,000 mile intervals.  The oil change, though easy and quick, is the single most important maintenance step that increases the lifespan of your vehicle...NO MATTER HOW OLD IT IS, WHERE ITS DRIVEN, OR HOW ITS DRIVEN.

Remember...the proper functioning part is only as good as the diagnosis and installation.