I've bought several Hermes scarves on ebay. There are fake ones out there, but the majority are real. Buying authentic scarves isn't nearly as tricky as buying handbags.
The fake ones lack the vibrancy of the real ones, and do not duplicate the actual designs, as they are nearly impossible to replicate since they are made with up to a dozen silkscreen screens. Instead, the copies are more "inspired" designs and are easy to spot in a clear photo.
So get to know the designs - there are about a dozen new designs each year, and get to know the colors they come in. Almost any design you want will come up eventually. Look for auctions with clear photos. And make sure there's a close up of the edge of the scarf. The edge should be rolled and still puffy. Never, EVER iron the edge of an Hermes scarf.
Another good thing to look for is that the scarf still has the original fold lines in it, indicating that it's never been worn. It's amazing how many people get these scarves as presents, never wear them, and end up ebaying them. I consider this the most desireable condition.
Otherwise, just pay attention to condition, and if the seller doesn't state that it is without stains or snags, write and ask about them. Ask whether the scarf has been drycleaned too. It's preferable not to clean them, but if they must be cleaned, Hermes recommends that they be sent to tiecrafters.com in NYC.
The truly vintage scarves have a softer silk twill, and aren't as desireable, in my opinion. The ones made in the past 15-20 years are nicer. In addition, the newer designs are more crisp than the traditional designs, as those screens have been used so many times that they're just not as exact, and often not as detailed as the newer ones.
If you have questions about any particular scarf, post it here or PM me. I'm currently saving for a handbag so there's no chance I'll be bidding on a scarf you're interested in.