Tell him that he's taking equivalent of an engine out of an out if production car.
- it'll cost him money to take it out
- it'll cost him more money to pack it, ship it, store it
- new homeowner won't be able to use the system without it, which in some states is illegal (to show working house and disable it after the sale)
- most importantly, he might not be able to use it at new house because he won't have a complete system, new dealer will charge him more to try to piece it back together than it would cost to just buy new.

On the other hand, you'll make more money removing old gear for old homeowner and installing new gear for new homeowner