Your buyer may have been a bit lazy in viewing the photos, but as an eBay seller, you are responsible for clearly and fully describing your items. It wouldn't take long to list the exact year of each coin in your description, and would have saved you from this issue, even if the buyer had filed an Item Not As Described ("INAD") claim, which he might well have done successfully in this situation.
Consider this a learning experience and work on making your photos and descriptions more complete. Fortunately the buyer didn't leave you negative FB, but as the others have said, it's really the DSRs you need to be concerned about. This is particularly true when you're just starting to establish a presence on eBay as a seller. A couple of mistakes like this (and/or a couple of INAD claims against you) could have a disastrous effect on your ability to sell for some time.
You mention on your "Me" page that you bought the "eBay for Dummies" book. It's worth reviewing the section on preparing listings, however, be aware that things have changed radically on eBay in the past year especially, so almost all of those "Dummies" books will be hopelessly out of date when it comes to specific eBay policies.
Review the "best practices" and "selling" pages on the eBay site itself (which is a lot more up to date, although not entirely so). And scan through these discussion boards from time to time -- you may get information on how to avoid issues that others have had to deal with that you may not have even been aware of.
