Just so you know, eVGA offers a step-up program in which you can trade in your original card for a better one up to 90 days after the original purchase, and all you pay is the difference in price between the two cards. You can't do it more than once for a given card, and they'll only honor the step-up if it was bought directly from them or from an authorized retailer (listed on their site). Go here
to learn more.
In three months mid range DX10 might not be out, but it would be worth it to get an eVGA card in case step-up turns out to be advantageous.
As for eBay, it's impossible to say how much you'll get, but I'd say around what you're seeing now, depending on how informed your buyer is.