Hi, where would be the best place to advertise a very good quality, higher end (County) used saddle for sale? TIA! 
Also list it on Dressage Star and Tack Trader. The online sites are generally free or close to it, so cover a bunch of bases. I’ve sold two Countys over the last few years; one went on DS and one on TT. I did get some spam responses on Dressage Star but they were easy to pick out. Post one or two pictures, have some others available to send to interested buyers and have every measurement you can think of.
If your gmo has a newsletter, use that too! And - you can always put it on eBay, listings are free - if it sells, great, if not, it just cost you a bit of time to list it. I have sold a few over the years, and find it is a bit of a timing thing - if someone happens to be looking for your saddle, great, you make a sale.
I (as well as my trainers) have sold a couple of saddles through some of the Facebook groups! Dressage tack/horse sales is probably my favorite, but I’ve sold several other things (though they weren’t discipline specific, like blankets and Joules polos) through English tack trader and english horse tack/supplies. It doesn’t hurt to try ebay either–I put my Petries on there as a last resort (not really expecting them to sell) and they did! I would also contact some of the other trainers in your area/the gmo and if you have a regular saddle fitter that comes to your area often. You never know who is looking, and it doesn’t hurt to send them a quick Facebook message (just sold my horse that way!) You might also want to check with any consignment shops in dressage heavy areas (I think there’s a big one in Welly?-- since so many people are down there right now!
I have sold saddles on Ebay. I don’t have that great of luck on Facebook, but I cover my bases there as well.
If you are buying a new saddle through a saddle maker, ask them, sometimes they will take trade ins. Voltaire does this for fine used French saddles.
Redwood Tack and Maryland Tack Exchange will buy your saddle outright, if its a model they need. I love Redwood Tack, Leah is wonderful.
