I flogged my old system to a mate for exactly half of what it was then available for at retail.
Suited him and suited me at the time. If flogging it on eBay or Facebook marketplace I'd probably have asked a bit more (2/3rds?)
True, that's why I started out by seeing what the equivalent bits were available for brand new as a good starting point. That does of course require exact makes, models and specifications of the panels and the controller, and a list of what extras if any are included.
I just sold the panels and the controller, but threw in free the resettable circuit breaker as it would have been too small for my new system. Helped him install it too, but then he is a mate.