You may want to get a buy bid from Carmax. I'm strongly considering getting rid of a C350 my wife drives, she wants an M3. I had a trade bid from the Mercedes dealer we bought the C350 from, looking at the CLS550. I went to Carmax last weekend and got a bid from them for the exact same amount.
I'm considering CPO cars, but there are none around here and not that many desirable ones for a decent price anywhere. With the loyalty rebate, club rebate and invoice+ pricing, it makes more sense to get a new one.
Our local dealer is fairly stingy, but was able to get them to agree to an amount that makes it close to ordering a car from Ryan or another distant dealer and shipping it. PCD appeals to me, but it gets very expensive for me to take days off, so it's not really worth it.