Fair market value: this can be tough, and I know that's what you're trying to do here. Start off browsing eBay. Don't look at the active listings, but the completed listings. See what similar saddles sold for. Saddles will sell for more if you can sell it locally than on eBay, so the value you figure out from this should be on the low end. Based on this, figure out what is the minimum you'd be willing to sell it for. Now you know what you WANT to get for it. Put the price you want in your ad. Be prepared to negotiate down to your minimum.
Know every possible detail about your saddle. In your ad, be sure to have the correct seat size, bar-size, type of saddle (barrel, roping, etc), and any other measurements or details that could be helpful. Provide a picture straight from the side, straight from the front, from the top, and from the bottom. Know everything else about your saddle: any flaws, manufacturer, type of leather, model name or number. You won't be able to figure out everything, just know as much as you can. If people ask questions about your saddle, you want to give them an honest, accurate answer. It increases your credibility, and hence the buyer's confidence in their purchase.
It's a very pretty saddle. Take some good pictures - clear and in good (flattering) lighting.