Dude, you probably won't get 400 for the two, they are not worth 200 individually, my friend has a 5850, that was 250, before that he had a 5770, that was about 180. I would price them more around the 200-250 range as a pair.
More people would be willing to buy just one card too, not two cards. I would put them as 150$ each but then 250-275$ for the pair, that way it makes people think it's a better deal. You will get way more bites on it too. You could even list them with just a general price and say you'll accept the best offer. Just some future advice. Good luck selling them.