No idea - maybe they get enough takers that they don't think it's a problem. Who knows? I tend to agree with you though, I think maybe some explanation along with the offer would be nice. They may just not care one way or the other though. Again, who knows?
Remember that people sell camera equipment for a variety of reasons. Estate sales come to mind. I wouldn't take it personally, they are just making an offer based on market demand and the potential for needing to warehouse the gear for a while.
Speaking as a buyer on a budget, I'd suggest trying to sell at a price point that is 25% to 30% less than what you see on mpb ,keh, etc. You might attract a buyer. Someone did that on this forum and it moved immediately.
I just bought a very nice d7200 with 20,000 actuations and a grip, with 2 batteries for $300 from a local camera store. They had 2, the one I bought was priced at $460. A 2nd one was $299, with 28000 actuations but was rough. They offered me the better one at the lower price and I took it. I'm pretty happy with it except I hate no touch screen for live view work and checking photos.
I now have a backup body for sketchy areas, I can send my d500 in for service, and have a 2nd body that will work for landscape, and botanical work. It works with my dx 16-80 f2.8-f4. It also works well with my 55mm micro nikkor f2.8. It also works with my 500mm PF (bought used), 300mm pf (refurbished), and 150-600 g2 tamron.
I don't know what mpb or keh might offer someone with a d850 to sell. But I might have bought one in the $1000-$1200 range if it was low use and in good shape. A good used d6ii or d7ii are so close in price to good quality used D850s priced at $1800 that a buyer will probably head that direction.
I think I could end up with a z8, a d850 or z7ii, and keep my d500 and d7200 because I like them and use them and they won't be worth much. I'll probably slowly add z-mount lenses after getting the z8.
The used dslr gear is helping me learn and really enjoy my photography. On a budget.
But I still expect to move to mirrorless (z8) soon.
I thought it might be helpful for folks to hear from a buyer's perspective
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
. I'm quite respectful of what folks have put into the gear they are selling, but I am also mindful that I'm buying into an old system.