most electronics on a used boat have virtually ZERO value in the great scheme of things due to age, installation or lack of maintenance. Useful lifespan is what you, the captain or owner, need to safely use the boat.
As far as any discount on the sale price, you can reasonably posit that any gear over 5 or 6 years is obsolete, as compared to new gear. It may well be quite useful and serve your needs well, but repairs
and updates are likely not available, especially when compared to new gear.
For me, all I really need was an autopilot
, depth/wind/tridata instrument, and a chart plotter so the 15+ year old gear aboard was fine for my use.