Doesn't seem like a great deal. I got my '17 C63S Coupe last month here in SoCal, where people have too much money in general so dealers seem to give crappier discounts than just about anywhere else. Sticker was $88K, no damage, and I was able to negotiate both $10K off plus get them to take my trade-in for private party value. For reference, that means hey threw in about $3K more on my trade in than any other dealer was willing to give me. Makes me think I could have got an extra few thousand off the sticker if I didn't have the trade. Oh and it was a brand new car that just hit their lot, so not like it was on the lot for months and they couldn't move it.
If if you really want that car go back, tell them you're ready to do the deal right there and then, but that they need to take another $5K off the car. I'm sure they'll do something more, but I could be wrong.