The bank just said they use "The Blue Book".

I went on edmunds.com and put in all the info and this is what I got:

2005 Chrysler Crossfire SRT-6 Convertible
33,000 miles
Condition : "Clean"

Trade-in: $14,211
Private Party: $16,367
Dealer Sale: $18,131
Chrysler Certified: $19,363

The bank will only loan if the price is $16,000, since this is a private party sale. Maybe they just use edmunds.com, too.