A few thoughts on this...
First -- You have no idea what price you will get on ebay. Especially after a big deal like neweggs goes out, suddenly tons of the same item start popping up on ebay. This drives prices down. Don't expect to make the same price point on a product when it was scarce as when it is abundant.
Second -- 13%. This is roughly the minimum cut that Ebay + Paypal takes from a sale. To make your $200 back you need to sell for about $235 dollars (if you include shipping price in the sale you only pay the paypal %). At $269 you make maybe $30. Nothing to sneeze at, but still a gamble and still doesn't cover the price difference to the 6950
Third -- A 6950 unlocking to a 6970 is not guaranteed to work, and you need the 2GB version apparently.
So with all those warnings in place, I will say that the 6970 is certainly a superior card to a 5870 except for the ridiculous amount of power it eats. So it's your call, is a 15 - 20% performance boost worth the risk involved? Only you can say.
I'd probably try it myself though. If you have the patience to wait it out.