I would tend to argue that the dealer will not give as much as you could get on your own if your mind is set on it. If it is good condition, I wouldn't be surprised if the 8 hp goes for 3-4 hundred bucks pretty quickly. I think you would be hard pressed to get much more than 400 though. Sure sometimes you see them on the list forever for more, but they never sell.....
As for the main, you could probably get 1000 to 2500 for it depending on hours, cosmetics, and running condition. I paid 1500 a few years back for a good Evinrude 140 for my fishing boat in Spokane and was quite happy to pay the price for a decent runner.
Of course, these are just a just a guess and they are only worth what somebody will pay for them. I suspect you will get beans for them on a trade.....