Personally, I would only claim a bag was new if I had bought it from the shop myself, had never used it, had stored it very carefully and it had absolutely no signs of having been a used return, or anything.
If it still had the tags attached, I would describe it as being BNWT and in mint condition and if I'd removed them (which I often do to protect the leather), or it didn't come with any, I'd describe it as being BNWOT and in mint condition.
If I'd bought the bag from someone else, even if she had claimed she had never used it and I knew I had never used it, I would not describe it as new, unless it still had the tags attached and was flawless. Even then, I would mention in the description, that it had been owned by a previous owner, but that it had never been used and was in mint condition.
If the previously owned bag had no tags attached, even if it was in mint condition, as I couldn't be absolutely positive that the previous owner was telling the truth about not having used it; I would describe it as being in mint condition, rather than BNWOT and I would then state in the description that it had been sold to me as unused and that I had not used it myself and that it, therefore, had no signs of use and was in mint condition.
From my experience, I really don't think many sellers think like this, though.
Very sadly, many will happily describe an, obviously, used bag as new, let alone an unused bag that has been previously owned.
I think this is very wrong, on two levels, as not only is the buyer, almost certainly, conned into bidding too high for a bag in SNAD condition, but also, bags are not like most pieces of jewellery (for example), as they are, generally, not easy to clean and so, are second only to underwear and shoes in terms of many people, specifically, not wanting to buy used ones; so, to lie about condition can really upset the buyer.