If the space between the two panes of glass are clouded, then the window has to be replaced. If you know the manufacturer, sometimes you can just get the glass part of the window. Otherwise you would need to at least replace the glass and the window frame (but not necessarily the whole window, casing, etc)
It sounds like the winow seals have failed.
Most sellers do not have a pre-listing inspection - something most Realtors
wish they would do.
Get an inspection and you can respond formally that the sellers replace the
windows before closing - maybe they will.
The only real cure I know of is to have the windows replaced. I've heard some
companies claim to be able to pump a vacuum between the pains of glass,
add the gas and reseal. I have never seen that work.
Make sure you have your own agent working for you.