Like Juanmi said, Facebook has a lot of groups that buy/sell tack and you can usually find a group that is local. I know there is an app called “nextdoor” that you can post an add on and depending on the setting people choose it will show them. I don’t think I explained that well... for example, if I had the app, I could change the settings so only adds in certain areas will show up. You can do only your town, neighboring towns, further and further out. This is always a good idea so you won’t have to pay for shipping which is a pain. You can always ask around your barn as well. If you know people from other barns ask them if they know anyone who is looking for a saddle.
This isn’t the best answer, sorry! But I hope it helped!
There are several pages and Facebook groups where you can post your ad. eBay and Amazon are very visted (specially eBay for used things). Local tack groups of Facebook can also help as i mentioned before. Instagram can be an alternarive if you have several followers.
Hope your question was answered.