Please consider this guide the definitive guide currently on eBay.  The other guides out there to "help" you make a purchase are misleading and contain false information and are too brand focused/loyal to be subjective.

Common Misconception #1.  This should definitely be addressed first.  This is the most common theme among people interested in upgrading. 

I need the biggest and best hardware in order for my car to sound good.

Truth: Unless you are a serious audiophile, there is no need for the biggest or the best.  Brands like Focal, DLS, Digital Designs, CDT, Boston Acoustics and other very high priced brands are only worth the money if you are competing or just need to feel like you have the best.  Truth be told, most people considering upgrading their car stereo source, speakers, amplifiers and subwoofers would do just fine with almost any brand you can find.  These brands include Infinity, Sony, Pioneer, Kenwood and other popular brands.  These brand's entry level lines are to be considered quite listenable.  This applies to their amps, source units, subwoofers and interior speakers.  These selections should not be driven to 100% of their advertised capacity, but they will sound good at 50-75% of their advertised output.  The higher up the food chain you go, the more willing the items will be to reach their full capacity and you will begin to hear tonality differences at higher volumes. 

You will find many people who will say "Infinity Reference is crap, I got the Kappa's", but if you sat them in a room and A/B'd speakers for them, they likely couldn't tell you which was which.  They may even find themselves preferring the less expensive speaker.  Speaker selection is very subjective and you should really listen to speakers in person before you buy them.

This is not to say that there is no junk out on the market, because there is.  It's not the sound that's so bad, just the longevity.  As long as you stay with a brand you have heard of for years (like those mentioned above who make all types of consumer electronics), you will likely have a positive experience.  Examples of companies I would stay away from companies like "Dual", "Pyle", "Pyramid" and the like.  Some may have success with their products, but generally speaking, they are not of the highest quality and most people experience premature failure of these items.

Misconception #2.  I need to buy (insert brand name here) wiring/RCA's in order for my system to sound good.

Truth: There is no real world difference in wiring for the most part.  Copper is copper.  Strand counts make a difference, but the same sized strand/wire made by stinger, streetwires or audiopipe will all be almost identical.  Same goes for RCA's.  Buy whatever the lowest priced RCA's are that you are comfortable purchasing.  As long as they are shielded in some way, they will be just as good as your buddy's uber-expensive twisted, purple, "titanium" RCAs.  He'll tell you otherwise, but he'll be wrong.  Do some research for yourself on the internet regarding this subject and you will find that the experts all agree ... copper is copper.  Use your common sense and think about it, don't listen to your friends telling you that you HAVE to have Streetwires or else it's no good.

That said, if you use THE best of everything (see Earl Zauzmer's BMW from the early 90's), you may hear a difference from using ultra shielded, oxygen enhanced, blah blah blah wiring, but if you have any regular consumer grade equipment, your ears likely won't notice any difference.  If you have noise, you have a problem that will be ground related 99% of the time - RCA's are unlikely to be the issue.

Misconception #3.  Used car audio equipment is too much of a crap shoot.

There is a little truth to this statement, as it can be a bit of a crap shoot if you're not doing research, but if you know what you're looking for, find a seller with over 100 feedback and 100% positive.  With that combo, you can be pretty sure you're going to have a good transaction and get gear that arrives as advertised.

You can save a TON of money buying late model car audio equipment and have no problem with it.  This goes for amps, subs and interior speakers.  Source units can be cheaper as well, but you probably want the latest and greatest features when buying a source unit (mp3, bluetooth, USB, DVD, etc), so depending on your needs, you may not save as much on a source unit.  As always, be careful to look for auctions that are detailed, many pics, receipts, complete with all accessories ... this hints to the fact the piece was well taken care of.

One purchase that stands out to me was a pair of subwoofers I bought from eBay that were approximately 4 model years old and well over 6 calendar years old.  I got 2 10" subwoofers for $50 shipped.  I built a very nice box for them and when I powered them up, I was in audio heaven.  I wasn't going to win any SPL competitions, but they really did have nice low end extension and great punch.  This leads me to rule #4.

Misconception #4.  I need the biggest/best/latest/greatest subwoofers in order to get serious bass.

This is absolutely false.  The box means more than the sub.  Plain and simply put, the box makes more of a difference than the sub.  You could buy a generic, cheap paper cone sub with a foam surround from an online parts store and build the ideal box for it after sizing up the theile-small parameters and you'll be shocked at how great it sounds.  Now, the materials the sub is made with will also affect it's longevity, so paper and foam isn't your best choice if you're looking to keep them for more than a year.  Not to say paper and foam can't last longer than a year, but you're more likely to get longevity out of some type of molded cone and a butyl surround.  The car environment is a bit harsh after all ... huge temperature and humidity swings.

This is the rule most people won't agree with, but it's true.  Look at how successful the original Bazooka tubes were.  If you take a close listen, they are actually quite acceptable as subwoofers.  Truth is, the driver they use is complete "junk" on its own merits (stamped basket, paper cone) by most people's assessments, but that enclosure is specifically tuned to the sub.  The result is great bass.  You won't win SPL competitions, but that's not the point of 1 10" subwoofer.  If you want to slam, get some 15's.

Misconception #5.  I need 4 way coaxials or component speakers for the rear too.

Truth: You don't even need to upgrade your rear speakers.  Rear speakers in a car should act as "rear fill".  What this means is that they should be muted when compared to your front speakers.  When seated in the front seat, you should barely be able to make out sound coming from behind you.  90% of your sound should come from directly in front of you.  Therefore, why spend money upgrading speakers you will rarely (if ever) hear.  If you want to upgrade your stock speakers, the entry level brands discussed above would be a good choice.  Do not get component speakers for the rear.  Some of your friends may be impressed, but any audiophile will simply laugh at you.  If anything, skip the tweeter and just get some nice midbasses for the rear.  The lower the frequency of the sound, the less directional it is and the less likely your system will be rear-balanced.

This can even be true when setting up a 5.1 dolby car, which is now possible.  About 15% of sound from any movie comes from behind you.  You may benefit from some of the entry level brand speakers here too, but there would still be no need for component speakers in the rear.

Misconception #6. My lights are dimming - I really NEED a capacitor!

Truth: Capacitors are a waste of money.  OK ... this isn't totally true, but for the most part it is true.  MOST systems do not require the use of a capacitor.  A capacitor will not stop your lights from dimming ... if you want to stop your lights from dimming, get a second battery (deep cycle or gel cell) and a battery isolator.  If that doesn't work, get a new alternator rated for a higher amperage.  Both options are relatively expensive, but worth it if your dimming is THAT bad.  In the past, I have experienced very SMALL relief from using a capacitor, but the fact remained that my lights were still dimming.  You might also find some help with dimming by upgrading all grounds in your engine bay (including the ground for your battery and alternator) to a higher guage wire.  People sell all in one "grounding kits" which work, but really all you need to do is buy some 2 gauge or 1/0 gauge wiring and some ring terminals and re-ground everything to the chassis.  This can also help with ground loops.

Source Units: A quick note about source units: these are highly subjective.  You need to find the options you want (mp3? Sirius? bluetooth? XM? video? DVD? iPod compatible?) and the ergonomics you prefer.  For instance, I have used Alpine most of my life, therefore I prefer their button layout.  However, some have been using Kenwood all their lives and prefer their ergonomics.  If you're open to any brand (which you should be), go to the stores and play with them, find the one that suits you best.  This is one instance where spending good money on a good source unit that has everything you want/need is a pretty good idea.  Don't pay money because of the brand, pay good money for the layout and options you prefer.  Used source units can be had at very good deals here on eBay, but keep in mind the seller qualifications you should be looking for.

There's lots more to all of this.  Car audio is a true science that takes years to learn and forever to master, but the above article addresses some of the most common misconceptions I hear everyday.  Please do not ignore any of this guide you don't understand ... there is plenty of information on the internet to support these claims, do some Google searching and learn something new.  The only people giving this article negative marks are doing so because they have been misled for so long they believe their view to be the truth and refuse to research on their own.

I hope this helped the rest of you and good luck building your system.