I'm actually going through my G3s right now. What I did was I pulled out ALL the ponies. I started by dividing by color (pink, purple, blue, green, yellow, orange, white) and then went through each color and put them in piles of the same pony (all of Rainbow Dash together, all of Cheerilee together, etc.) Then I looked at each small pile and decided a) do I want any of these? b) if so, which ONE. Then I boxed up everything that was left over in boxes labeled "to be sold" so that I know I've already gone through them and made my decision. The G3s I'm keeping is down under 20 (and I started somewhere around 300). I just have to find time to take pictures and post a sales thread now. I've decided that the condition they're in is the condition the buyer gets it in. They can clean it up to their liking. Of course, since my goal in getting rid of the ponies is to get rid of them, they'll be cheap so I don't feel bad making them clean it up.
If you're planning to match up the accessories with the ponies (most of mine came from thrift to begin with so I don't have the accessories) then you might want to ID the accessories first and then find the ponies to match up. The other option is to list the accessories as separates, in a lot or as several lots (brushes, ribbons, pony wear, etc.) and let whoever buys them figure it out.