wrote
If the new owner wants new air it's HIS problem to fix it, as long as
you disclose that the central air is "there and functional for the
time being". (Assuming) You are selling the house at the price it
would sell for with no AC so if it works it's a bonus.

That is true, but. . . . . .

How does it really affect the selling price? Let's say the new unit is
$6000. What is the difference in selling price of the house with the old
unit versus a brand new energy efficient unit? Can you get your money back?
Many people seem to want to just move in and have everything perfect, not
have to lift a finger. I've watched a couple of TV shows where people
rejected a house because of ugly paint in a room. For $15 and an hour
labor, you get the color of your choice.

Personally, I'd low ball the bid by $10,000 and choose the brand and
installer of my choice but some non-mechanical types just want to set the
thermostat and be cool. That 20 year old unit can die tomorrow, but it may
go another 20 years too. Bit of a crap shoot.

Any realtors here have a comment?