Craigslist: Great because people can see it, touch it, feel it, bring their wither-tracing or their horse, or you can agree to hold their cash while they fit it, etc. You know your customer will be happy. Free eBay
: Great because you can reach customers all over the world. Doesn't tend to fetch the best prices. An unhappy customer can reall be a pain if you haven't defined your return and trial policy very carefully. 9% fee
Horseclicks: Great, because it's free and reaches a lot of people. Easy to search and find what you need. Free
TackTrader: Similar to Horseclicks, but tack only. I find it cumbersome to search in to find what I'm looking for. Free.
In all of the above, be sure to describe the saddle thoroughly and accurately. That will cut down on annoying inquiries that aren't serious, and reduce the chance of an unhappy customer and return. Beware of flakes. Make sure you get cash or Paypal, trial/return agreements in writing, etc.