I will echo a couple points others have made. Typical retail markup is 40%. So that 400€ BCD cost the seller 240€. So the seller is trying to fully get their investment back which is not reasonable after 10 years. One needs to cut their losses and move on.
While the item is new in the box, one does not know how it was stored. Storage impacts the life. Also after ten years there is no warranty and their may not even be replacement parts (bladder). At least if one bought it via eBay there would be some buyer protection.
At the same I understand the OP wanting the same style so there is no learning curve. As such,I could see making the seller an offer of 100€-120€. With the caveat as suggested they show the BCD fully inflated. I would go so far to ask that it be inflated with OPV being exercised. That would require a video to be sent.
I would argue that, realistically, it should be
either a lowball offer,
or a request for video to be made, not both. With the video, one would effectively request proof that the product is functioning as-new, and with no wear and tear as well, which would make the offer less justifiable.
Thinking more about it, I would lean towards making a reasonable offer, maybe 120€, expecting a counteroffer of 150€, then if something is wrong - use the old BC for parts. Inflator, hose, and deflators are easy to swap. Bladder I'm not sure about, but it's likely the same size as well.
It would be a decent purchase for 120€-150€ I think.
UPD: whatever you end up doing, don't add those 10 kilos back!
I've shed and added similar amounts, gets increasingly difficult with age.