The models you cite are upscale cars (i.e. E500) that, like trophy wives, are costly and sometimes suddenly lose their appeal. So if that's what its to be, know what you want and be ready to act - to bail out some guy who suddenly needs cash. You're not going to be Mr. Nice Guy but rather Mr. Robber, in the seller's opinion. Your attitude should be "so who NEEDS a SLK?" I'm here and have the cash (you need). Take it or leave it.
Dealers are quite flexible, especially when a car has been on their lot for 90+ days. A CPO Warranty is worth something and that ought not to be overlooked. Like BMWs, M-B are costly to have repaired.
So buy privately at a low price if you're a risk taker, if not, pay a dealer for a warrantied car.
Sell your BMW privately if you have the time to do that. Have its maintenance and repair records and a recent CARFAX to attest to its quality and history. Be realistic about its price so you don't waste YOUR time.
I believe dealers make their money trading in used, rather than new cars because there's no MSRP on a used car and the $$ are made on the auction vs. retail spread. A dealer will offer only auction value -- why should a dealer pay more for a trade in?
Hope this helps you.