If you are certain of the names of all of the ponies you can skip this step, but if you're not sure, posting group pictures of them here would probably get you plenty of help in naming the ones you aren't sure about.
Otherwise, what everyone else has said. List the flaws, take good pictures, especially of any flaws. And if you aren't sure what kind of price to put, its fine to ask. People will have differing views, but that ought to give you a good average to work from.
You also need to work out what your sale range is. If you are only shipping to your own country, you may find that you have fewer buyers, but your shipping might be simpler. The Arena is international, with representation from most countries around the world, so that's a key consideration. If you are shipping internationally, it's worth knowing if any ponies you have were not sold in other countries. It's possible you might make a little more if there are more people interested - but checking out shipping costs is key!