I wouldn't restore the system to "factory settings." If the buyer turns out to be in any waters where it's illegal to discharge overboard, he'd just have to remove 'em...if turns out be be in waters where he can use overboard toilet and/or tank discharge, he may want to configure things entirely differently.
So my advice: Do only two things: Replace the hoses. Because rubber and plastics dry out over time, becoming hard, brittle and prone to cracking and splitting, 10 years is the average working life of any hose and yours are on the cusp of that. If the toilet is a Jabscomanual, replace the pump assembly--it's easier than rebuilding it and only costs about 20% more than the kit. If it's a macerating electric toilet make sure it's in perfect working order. If it's a Jabsco, it might even be worth the $150 to replace the pump motor assembly.
You'll spend a bit more than the cost of a y-valve, but to a buyer all new hoses and a "new" toilet pump assembly on an older boat isn't something they're likely to find on any other boat and should be worth a bit more than you'll get without 'em 'cuz it's one system that won't cost them any money for a few years, So I think you'll get most of it back.
--Peggie