This is a tough one. I think it ultimately comes down to how much work you are willing to put into her. I personally think you did overpay for her condition, but if she cleans up better, then it would have been worth it.
I think it would be very wrong to ask the seller for a partial refund, given this seller was obviously someone who is clueless about ponies. I think you and all the other bidders took a chance on the pony with the given pictres and information.
So, if I were in your shoes, I would either keep her and work with her, or return her for a full refund. I would not do the inbetween, but that is up to you.
I do feel for you on this though. It is a tough situation!