question re BMW purchase. A case?

A little over a year ago I purchased a BMW Z3 from a private party. I have to admit, I was very hesitant to do so, especially considering the fact that I wouldn't have a dealer to fall back on should anything go wrong. I was also told by the seller that he had a little work done on the door panel due to a minor incident that he had while driving it. Well, this definitely sent up a red flag and I did all my research on the car prior to the purchase. I got a copy of the carfax report which showed nothing negative...not even the body work mentioned above. I even took it to the BMW dealer and paid for them to perform the "pre-purchase inspection" on the car. I specifically told them about the body work and asked them to take a look at the frame and report any damage that they see. Well, not only did it pass, I even had the service manager tell me that I was getting a heck of a deal. After hearing that and considering the fact that I still had 15K left under warranty, I went ahead and bought the car. I did have the seller sign a waver stating that the sale of the car was contingent on the car passing the BMW certified pre-purchase test so I felt confident that I was getting a good deal when the car passed the inspection.

Recently I took the car to Carmax to see about trading it in on something larger and, sure enough, they spot some frame damage on the inside portion of the door jam. The inspector came out and showed me the weld marks (and rust) where it was obvious that work was performed. The offer that they gave me was ridiculously low and would leave me with about 10K in negative equity should I decide to trade it in.

I was wondering if anybody out there has had a similar experience. I don't think that this falls under the "Lemon Law" but I also feel that since the BMW dealer signed off on a a pre-purchase inspection that they should be held at least partially accountable. My purchase was based on the inspection of a certified BMW dealer. They are the "experts" after all, correct?