Re: Grey water
If you have made no modifications to the septic/gray water system since you purchased the house, you are under NO obligation to change it now.
The way MOST inspectors do their inspections is to call attention to everything, so as to cover their ass, whether said item needs attention or not. If they cannot physically put eyes on something, they note it as such, which is what has happened in your case, the inspector can't see what's going on with the gray water - if anything - so let's just assume that it's faulty.
If the buyer wants to pay for an in depth inspection of the septic system and base their purchase on that inspection, so be it, but you are under no obligation to change, modify, amend, or otherwise do anything to your home, just because a buyer says so. Instead, take this opportunity to thank said buyer for their interest, with the added warning that the door not to hit them in the ass on their way out.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!