Perfumer Jeffrey Dame said this in an interview:
...A scent made within the past year will be brighter in top notes, a scent 2-3 years old is mellower, as everything melds together. A scent 4-5 years old starts to lose character, and can often be turning bad. Anything over 5 years old is highly suspect for quality, and anything 8 years old is dreck and turned. Buying any fragrance on Ebay over 10 years old tells you nothing, as you either end up with fragrance which is completely unwearable, or even if it is wearable, bears no relation to the original scent when it was first made. Fragrance goes bad...
Scroll down on the link for the interview:
Firstly, I am not a professional and this man has forgotten probably more than most of us here will ever know about fragrances. However, I am of the opinion that not ALL fragrances go bad. I think that if stored properly many can retain their integrity. The big question is how it was stored which is totally out of a buyer's hands. Then again, I am not a professional and do respect his opinion. Maybe he knows something we don't.