I'm wondering which is of these packaging methods can be considered as "brand new never opened" and which is considered a value added by the vendor.
In my experience (which is significantly less than many others on this site)....
When a mini is brand new from the manufacturer it is protected in some way. It is either lightly shrink wrapped of it comes in a re-sealable plastic bag. Victor releases are often shrink wrapped.
Some of the re-sealable bags have obviously never been opened (Warner is a good case in point). It's obvious because it's hard to open and the glue tends stick to both the flap and the body and "strings" and stretches between the two surfaces as it's opened.
Most of the re-sealable bags I've experienced do not stick like Warners (thank God) and they do vary in size and snugness.
In summary, the only ones that could strictly be called sealed are the shrink wrapped ones, the obvious Warner ones have never been opened (well, maybe once) and the others - anything's possible. That said, it could still be shrink wrapped after being opened if you have the right machinery.
I have minis that are "brand new never opened". I bought them from HMV Japan or Disk Union and have not got around to opening them yet ... but do you trust me?
What it comes down to is (a) if it's from HMV Japan, CD Japan or Disk Union then it's as good as you get (depending on transport damage of course) otherwise (b) get to know the sellers.
Most Japanese based large feedback 99%+ are extremely honourable people and to be trusted (most... there is one who words the auctions very carefully to omit the fact that the unique items they sell are illegal and of their own construction