Here's some of my favorite jersey sellers on Ebay
This guy sells quality authentics, not always a huge selection, but quality stuff and decent prices. You're not going to see authentics for $30 here, but then again that's exactly what you don't want. Authentics that go for super cheap on Ebay are usually irregulars, trust me, something is probably wrong with them. This guy usually has a couple of Skins jerseys, usually Taylor, Portis and I've even seen him selling authentic Cooleys from time to time. I bought an authentic white Arrington from him.
http://search.ebay.com/_W0QQfgtpZ1QQ...Zmedinafreddie
This guy has a wide selection and again, not at super cheap prices but still very reasonable. I've got a Theismann replithentic from him.
http://search.stores.ebay.com/search...&submit=Search
I bought a Doug Williams Mitchell & Ness from this guy
http://stores.ebay.com/REAL-DEAL-SALES
If you're looking for throwbacks on Ebay, stay away from any jerseys made by "Players of the Century", they are cheap knock-offs.