If you own the car and are selling it to the dealer (or trading it in) the dealer offers you a price for the car as is, no expectation that you will cough up anything that wasn't in the car at the time the dealer checked it out. That includes jackes, floor mats, spare tires, owner manuals, and EVSEs. That they are used to gas cars and may not realize a new owner might want the EVSE isn't your problem.. If it isn't in the car they shouldn't expect it to show up after the sale.
If you are returning a leased vehicle you are required to return all the equipment that came with the vehicle when you leased it, you don't own the EVSE so you can't sell it and expect to not get dinged for not returning it.