I'm a little too new to tell a fake from the real thing, so I avoid ebay right now. However, if you do a search on this forum, you will find a list of good ebay sellers.
Right now, I buy my fragrances from www.bigdiscount.com or www.scentiments.com. Both can usually rival (or beat) ebay sellers prices (from what I've seen anyways), and they're both reputable sellers with good customer service.
Hope this helps!