Hi

Buy in a shop inspect real carefully, pay premium.
Buy from an on line dealer with a reputaton and a written policy for returns, pay premium
If you buy a slightly fogged lens online then it may not clean up, it is very expensive to ghave reground and recoated, and may not be practical if it is a cemented pair...
Some lenses fog over after a few years, however they are stored - ask for quote/estimate and turnaround before you send for repair...

Noel