I always take such descriptions with a grain of salt. However, that being said, that boombox is rare enough in used form but to have one complete with all the packaging and accessories brand new, which are rarer by many orders of magnitude than the boombox itself and the price it sold for was certainly reasonable in my opinion. Some of you wouldn't be interested in that model, and perhaps don't think any box is worth more than $50, but that doesn't mean it's only worth $50 and more importantly, it's also why you didn't or wouldn't win it since someone else would regard that box much higher than you.
As for BNIB boxes being just like a used one, I would disagree on that note. I have a couple that are NIB (or at least advertised in that way when I got them) and they look, feel and sound like they are brand new, without all the glitchiness associated with older machines, controls smooth as glass, etc. Lots of us got into collecting these for sentimental reasons. When all else is broken and failed, what would it be like then to be able to open and put into service, a brand spanking new M90 or M70 or M9994 or (whatever boombox is your personal grail) 50 years later and be able to experience all that again -- priceless if the sentiment is important enough for one to re-experience all over again.
Anyhow, as far as keeping an item sealed or not to retain BNIB status... therein lies a dilemma. Would any of you be willing to shell out $3000, $4000, or even over $5000 or more, to buy a carton that has never been opened before? Because either the seller opens and photos it for documentation, or YOU will open it out of curiosity when you receive it. Either way, it will no longer be sealed BNIB. Unless of course you keep it sealed, but then how do you know that someday years later, you won't open it to discover a carton full of used telephone books? I would say that while keeping a true BNIB item sealed will be better for a buyer, the potential lower asking price due to a buyer unwilling to pay bucks without knowing what's exactly inside the carton makes it better for a seller to open and document the contents.
Anyhow, that's my humble opinion.