WBAC as with any other broker will hardly ever give you the market value for a car, house or what ever they're offering to sell for you. They also have to make money off it to pay for the advertising and the wages for people working in the company.
They'll offer an attractive price but when you actually try to sell it. They'll be taken money off for faults found plus I would guess some type of broker fee, so you'd get hundreds less than you were first offered.
It's only worth using a company like that if you're not able to sell the car yourself because you don't have the time.
As for the drop in value of the Exeo, this could have been number things. But to list a few your car is coming up to 5 years old which makes it less saleable. There is a growing anti diesel feeling that'll hit the value of all diesel cars. The Exeo is also caught up in the VW recall with questions over the long term reliability of the software update. Plus as said above the Exeo was never a popular car in the first place.
A couple of years ago the Exeo was at the top of list of cars to replace my
MK1 Leon. I still like the car but now I'm more likely to get be getting a
MK3 Leon once my MK1 stops working or tax increases force me to change it.