To begin with sell a few groups of ponies that you can easily live without. And then see how it goes, you don't have to get rid of them all. As someone said put them away for a while and see if you miss them.
Here is the best place to sell them, arena members who buy your ponies will give them good homes
Good luck with your sales (if you go through with it)
(I'd keep at least one pony to remind me of happy things like rainbows)