I don't know how much you're looking to sell them for, but I've had luck advertising pony bargains ~$3-4/pony here on the sales forum when I wanted to clear out some of my extras. They ranged in quality so some people got some real bargains. I've also picked up a few on the sales forum here in similar "bargain sales". People tend to buy several at once so they go fairly quickly. If you do this, I'd obviously price some of your rarer ones higher, but if you're mainly looking to sell common, non-mint ones maybe that's not an issue.
Good luck!