To me it seems like you're getting the worse deal, but I'm from the UK and think in pounds so +350 seems like a lot of cash difference. Go on ebay and compare selling prices of the two things (completed auctions, you should be able to find the same specs). Then see what the cash difference is. I personally would be inclined to haggle him down then also, ebay prices are a bit inflated.
Why are you doing this? The iMac is a great system.