Hey guys, let me start off by saying there are some really good jersey buying guides out there, and Ive learned a lot from them. But, none of them are as comprehensive as this one.
First things first, you're gonna want to familiarize yourself with the official Reebok made jerseys. The NFL store has some very high resolution pictures that allow you to zoom in and really see the finer details. You'll also notice they have three lines of NFL jerseys.
Replicas for $80:

Mesh Torso, non-mesh shoulders and sleeves. Numbers and names are not sewn on. Back and front are same length.
Premiere EQT for $110:

Mesh Torso, non-mesh shoulders and sleeves. Numbers and names are sewn on. Back is longer than front.
Authentic for $230:

Mesh Torso, shoulders, and sleeves. Numbers and names are sewn on. Back and front are same length. Better image can be found
here.
You'll also notice, none of them look like this:

The Bears dont play with a logo on their jersey.
Also, the sleeves are wrong, look:

Or here:

Its supposed to be 3 orange stripes with white borders:
Also, dont buy jerseys from sellers that use a picture like this:

The seller just snatched the picture from the NFL online store. If the picture is just stock:

Then theres a good chance youre gonna end up with something like this:

Listen, if you just remembered what you've learned so far, you'd have already saved yourself from 95% of the garbage out there. But, there's more.
These numbers are mesh, not sown on:

That means its a Replica (or just a really crappy knockoff).
If you cant spot the mesh:

then get your face really close to the screen.
Speaking of crappy pictures:

What is this? Some guy taking a picture of his closet? How are we supposed to see anything. Contact him and ask him for better pictures before buying.
Watch for colors:

Current Orange jerseys have white numbers with blue border.
And heres a summary
JERSEY STRIPES NUMBERS
Home 3 Orange, white borders White, with orange borders
Away 2 Navy, 1 white, no borders Navy, with orange borders
Alternate 3 Navy, white borders White, with navy borders
Ok, we've eliminated the most offensive defects, so now we're going to move onto less noticeable stuff. Technicalities really.
Here's a technicality:

The back of this jersey is longer than the front (this is normal for EQT jerseys).
Also, the numbers should be smaller on the front than on the back, unlike here:

An Authentic Bears jersey is supposed to have mesh throughout, unlike this:

Notice the different shades of blue? The torso is mesh, and the shoulders are not. Its easy to spot cuz the mesh isnt as shiny.
Heres a close up:

Now, none of that is a big deal. This is also normal for EQTs. A $110 EQT has a mesh torso, and non-mesh shoulders and sleeves. The Authentics have mesh shoulders and mesh sleeves.
A (poor) example of a mesh sleeve:

The stripes are not supposed to be meshed! But kudos for for the elastic sleeves and for not running the stripe through ol' GSH there (that's another common flaw).
Alright, time for a recap.
1: No team logo!
2. Watch the colors!
3. Dont buy from sellers with crappy or stock photos.
4. Number's border should border the entire number (not mentioned before, but figured it was important).
These apply to just Authentics:
5. The front and back of the shirt should be an even length.
6. The front numbers should be smaller than the back numbers.
7. Have mesh shoulders and sleeves.
8. Sleeve stripes should not be mesh.
Time for a quiz. Name atleast one defect from each of these:
1.

2.

3.

4.

5.

Some Answers:
1. Has 2 Logos, no stripes or numbers on sleeves.
2. Has Logo
3. Numbers look stupid, sleeve stripes are wrong color.
4. Colors of orange stripes are inconsistent. Shirt is not navy. Oddly, the spacing between the sleeve stripes are appropriately asymmetrical.
5. Stock photo. Numbers look like a 3 year old drew them on...which would be funny if it wasnt also true.
Ok, hopefully you got a 5 out 5, cuz those were pretty bad. And God help you if you missed #5 (btw I found all these pics off ebay being advertised as "Authentic").
Alright, lets move onto some very techincal stuff that should only concern the real jersey snobs out there.
Elastic sleeves!

floppy sleeves

not floppy

floppy again
The difference is elastic sleeves or non-elastic sleeves. I suppose it is player preference, and therefore your preference too. I can neither deny or confirm if NFL Authentics come with elastic or non-elastic sleeves, because I have never paid $230 for a jersey. And I never will.
Another interesting bit, is sometimes the sleeve stripes wrap all the way around:

and sometimes they dont:

Thats odd, and if anyone has any information as to why, then be sure to let us know. I'd guess it may just change with year, weather, or player preference.
And if you followed this guide, then hopefully you'll have gone from buying this:

To this:

Good luck, and remember to trust your instincts.
Author's Note:
This guide was intended for ebay, but they have a 5 picture limit, so f@ck'em.
Thanks to everyone who's jersey guide I have read and learend from.
Special Thanks to everyone at the The Fantasy Football Guys message boards and the Chicago Bears Message Boards for their advice in the making of this guide.
Very Speical Thanks to the Chicago Bears for aquiring Jay "The Savior" Cutler.
Feel Free to Ask Any Bears Jersey Questions here:
http://boards.chicagobears.com/forums/thread/1578545.aspx