Saddles Direct, British Dressage classified section on their website, Horsequest, Free Ads, Adtrader, and your local saddler.....
If your farrier is mobile ask him if he'll put an add in his van too.
Be careful of scam buyers though!! I've just been a victim of that. Within 2 hours of putting my saddles on saddleseek.com yesterday I was contacted by scam buyers.
It has frightened me a bit, and I've taken them off the site, and I've since decided I'd rather let my saddler sell them for me, and although I'll lose money from their percentage, it's better than being robbed by pikeys, or receiving stolen cheques and losing the saddle.
I just tried saddles direct for mine but didnt get a good quote back but they did say would more than likely get a better price if sold privately so I am doing just that. May sell it to saddler if dont get what I want though!