I'm in the process of trying to buy a brand new GTD, I've used DTD, Carwow and Orangewheels to get some prices and I've been into a dealer to have a chat about a price for my part ex, a 2008 BMW 330i convertible and they had the cheek to offer me £6k, that's not even a competitive part ex offer. So I'm selling privately instead.
The brokers were offering about 9.5% to 10% off the rrp price so it seems you have a lot of room left to negotiate yet.
Goodluck