I would have to say right here, at the Arena!
I MUCH prefer selling on here over ebay or really anywhere else. First off there's no fees, which really can add up with feebay. Also, you can easily put every pony you have to sell into 1 topic on the board which makes it easier. Also because this is full of ponyfans, you're less likely to get some bad buyer or something which might be a risk at like instagram or FB or so. Plus, feedback here really works. Another good thing is if you're just thinning ponies but getting into something else instead you could trade too.
Stay away from lame photobucket though.
Good luck with your sales