I can't help you with the pricing, but I can tell you that I've been scouring ebay on and off for the last couple of years for "My Little Pony G3 Bay Breeze", and she never showed up (at least not when I looked). So she is indeed very rare, but since there are no other successful offers, it's very difficult to determine what her worth is, I think. And I can't be the only one who would be interested in owning such a rare pony, even if she's not a pony from the MLP generation of my choice.
If you really want to sell her, your best option could be to offer a "buy it now or best offer" option (if you plan on using ebay)?