A dealer won't give me any more than £8000 for my Audi A6 against a BMW Z4 - is he trying it on?

I have a 2005 55-plate Audi A6 Avant 2.7 TDI S-Line with full leather, Hi-line MMI, 11 months MoT and FSH with 90,000 miles. I have today been to BMW Stafford to test drive a 54-plate 2004 BMW Z4 3.0 SE...

Priced possession

I'm probably being naive but why is there a huge discrepancy between What Car? Valuations, “We Buy Any Car” valuations and Dealer valuations. I have a three year old BMW Z4 2.0SE coming to the end of its...