imo you shoud be between trade and porivate party. "retail' is a scam to support car dealers.
take original invoice price, subtract all incentives, then add 500- that is what the car cost someone in 2012. Now subtract $12k-15k.
Does it have a cpo.100k extended warrnaty?
You should get a PPI too.
Keep it as simple as possible...but no simpler.