Autotrader bods won't know what the system is or what its worth, ebay might be a better bet.
Also, have you asked ionics if they can do you a trade-in on the system? As you're downsizing, they might even do you a straight swap. They might even take the van and system off you, never hurts to ask.
Also regarding autotrader, remember the new reg is out in less than a week, people will be holding off a bit as the market is about to be flooded with part-ex's that everyones trading in for the new reg.