I believe the head unit is the same on all cars. They are just programmed differently. So it's not worth much at all I'd keep it just in case.
The amp can be had for £300-£400 new. So one over a decade old. I would think anywhere between £30-£150 is fair. Depends on the demand I guess. Which isn't high unless someone has one die on them or waterlogged boot. So it's worth current DSP owners to re-locate them somewhere where water won't get to them or seal them somehow.
Why are you selling them? What did you upgrade?
I have the DSP carver system and it's been a pain to upgrade it without getting rid of the amp like you have done but then I'm told all the speakers are very sensitive to amps.
I love the sound system, but I want modern features which the OEM lacks.