Tough question, Adam.
Reproductions are out there, and often less than honest dealers will try to pass them off as originals at higher prices. There are a few rules that really increase your chances of getting authentic stuff.
1. Stick to double sided posters at first. It is a lot more difficult to reproduce them.
2. Double sided posters tend to be a bit lighter on the back side to allow more light through. Look for a difference in color balance and saturation of the back side.
3. Stay with reputable dealers. The all poster forum has a section that contains good info on good and bad dealers here: http://www.allposterforum.com/index.php/board,40.0.html
4. Try to buy directly from the theater, or if that isn't possible develop a relationship with one of the employees. Posters are often given to the employees as perks, and they in turn will sell them on places like the local craigslist. If you develop a relationship, you can get first shot and also be able to request stuff while it is still on the walls.
eBay feedback is a joke. Often the seller will do anything to avoid a negative. This includes slandering the buyer as non-paying. As a last resot, they will sometimes refund money to an experienced collector so they can keep selling fakes to the inexperienced. A dealer named tloce is notorious for this, but he isn't alone.
There are other auction sites, particularly emovieposter.com, movieposterbid.com and heritage auctions that are reputable. They will research for you, and will generally refund if a mistake is made.