It can be hit and miss to be honest.
There has been a number of sellers appearing on ebay with shops selling fakes. They buy up the empty bottles and fill with who knows what.
so first thing is always look at the packaging very carefully.
Ask lots of questions.
Read the feedback.
There is no guarantee if the box looks good the bottle inside with pefect. Even if unopened a fragrance can explode. That means evaporate.
Bottle collectors are willing to push up prices just for the bottle with no real interest in the juice.
A favourite term used by sellers is rare. Always take this with a pinch of salt, as many fragrances are widely available.
Saying that I have bought a bottle of Caron Infini extract from 1970 for £12. There was a slight chip in the topper and the seller stated this, so no doubt put off the bottle collectors.
The fragrance is totally gorgeous.