The Jordan XI (or Retro 11) is one of the most sought after shoes in the Air Jordan line.  It is also one of the most counterfeited sneakers.  Here are a few helpful hints and quick tips to avoid bogus sellers online:

1. The carbon fiber on the sole.  The best way to describe the pattern of the c.f. is that each is squarish with slightly rounder corners that just very slightly touche the other squares.  Avoid a pair that are round or oblong shaped and do not touch the others.
2. The 2 and 3 at the back of the shoe shoudl have a space or gap between them.  Most fakes do not have this gap. 
3. The Jumpman logo on the lateral side of the shoe.  On 1995-96 originals, the fingers are indistinguishable from the hand.  On 2000-01 retros, there are four fingers that look like a "webbed" hand.  On fakes, the are fingers are too long.
4. The tag on the tongue says JUMPMAN JORDAN.  The exception is for the Black/Royal colorway, otherwise known as the Spacejam XI released in 2000.  On this model the word JAM is used in place of JORDAN.  Also, on the tag of the White/Black/Concord colorway the Jumpman on the tongue is concord (purple).  On most fakes, the Jumpman is either red or blue.
5. The box tag and production label are also useful aids in spotting fakes.  An authentic dealer will not hide either of these labels and will readily provide you with them.  The production date located below the bar code touches the bar code.  On fake labels, this is not the case.  Since retro 11's are more likely to be fake, here are some authentic production dates and factories:  000608, 000709, 000810.  Those made in China are LN3.  The ones made in Taiwan are FT1.
6. The insole of all Jordan XI's made in 1995-96 say Nike Air.  On 2000-01 retros, this is true only for the white/black/concord and the white/columbia blue colorways.  The black/red, Spacejam, and cool grey/medium grey/white colorway all have the Jumpman.

I hope this guide helps you in your purchase of Jordan XI's.  As a longtime shoe collector, I do not support the proliferation of fake products on Ebay.  The best weapon is to educate ourselves.  This is a quick guide.  There are a few visual guides by doing a little search and research on other sites which should help you avoid making a purchase you may regret.