I had such a laugh when I read the subject line....I realize you are asking in all seriousness with a need to know, but.....2/3 of my fabric is 30 years old and I am the one who bought it! So I did a bit of a double take when I read your question. I was thinking..."Goodness I hope so!"
All kidding aside...my fabric has been stored in a temperature controlled environment, not exposed to light (shelves in closets) and was the best available at the time. So MY fabric is definitely usable and some quite collectible. That being said, I do agree with others that there is some risk buying sight unseen. However, I have bought yards and yards of old fabrics off EBay and for the most part have had no issues. Just be willing to accept what you receive for what you pay. And there is the key! Don't pay much and it will all be good!
