My car had same issue when I bought it from used auto dealer. They determined it was a bad yaw sensor. They got a used one off ebay from an MB donor car for $50 (vs $800 new). Problem solved and ABS/ESP light off. Granted may be another issue but parts are available both new and used to resolve issue. $8k is a great price but can't imagine dealer trade is more than $6k.

http://www.ebay.com/sch/i.html?_odkw...lk320&_sacat=0

This is a simple fix, less than 30 min.