Depends what you're into.
Early crewcabs are 'somewhat rare' and this looks like a decent one (which is even more rare as most have been heavily used commercially). If it is good, and somebody wants it, I don't think £6k is stupid. If I had sufficient readies, I'd be interested myself.
There have been a couple of (admittedly late) crewies for sale by importers at around the £6k mark.
But you can't compare your Devon with it, you need to compare with other Devons of similar age, model and condition. Your Devon, unless pretty good, might well be worth less than that crewie.
Sorry if I'm stating the bleeding obvious, and no - I don't understand agreed valuations either.