I personally suggest just going in to a Skins store, buying frmo the SKins store online, or maybe from a Sporting Goods store. I honestly have bought dozens of jerseys on ebay and it is truely hard to tell what you are buying until it actually shows up. My advice is to buy the $75 jersey that the store sells, unless you have the cash flow to buy the authentic for $250. Another warning is that if you plan on buying the $250 jersey (or any current players jersey) be ready to hang it in the closet after a year with the free agency these days.
I will give you a few hints to look for in jerseys on ebay:
(1) The nfl logo on the V neck:

This one claims to be a Sean Taylor custom jersey and personally is garbage,
(2) Check for signs of it being home made :

This one claims to be an authentic Moss jersey for $49.99, and you can tell that the nfl logo on the V neck is not prefessionally stitched on like it should be.
Sometimes these jerseys also have big numbers on the back and a bit smaller size numbers on the front. Its hard to tell sometimes from the pictures but I have seen them and they do not look good in person.
Authentics usually also have elastic around the arm band:

$289.99 but is also guaranteed real.
Replica (much cheaper):

$74.99
My own personal opinion is to avoid a black jersey as well. Hope the info helps.