Did the buyer get a receipt? Contact number? If the buyer is 100% convinced it was said party, I would post pertinent details regarding the sale. I think it is within the public interest.
This is why people are resorting to on-board cameras, whether they're in a car, on a push-bike or lorry etc... driving like this is becoming more common.
They were driving over the speed limit and over-taking on the approach to a roundabout? If so, then I think they deserve to get their car smashed to bits - it's the only way some people learn..
http://bit.ly/1QaijLs