Personally, I think the post suggesting that it has to do with people buying the 375 H&H as part of a dream, in some cases, has merit. A person buys a rifle because they dream of going to Africa on a dangerrous gane hunt, ot to Alaska or Russia on a brown bear, and maybe moose, hunt and want a rifl that will handle it well. Over time, they realize that they may never get to fulfill that dream, for whatever reasons, and have found that they really didn't need the rifle any more, and it goes up for sale.
For others, the cost of the ammo, as compared to their 30-06/300 or whatever their main hunting rifle is, and the recoil is such, that they do not shoot, or want to shoot, that rifle anymore, and it goes up for sale.
For others, the desire for something new drives the sale of their 375 H&H for the newer Ruger, or whatever (i.e Weatherby). No, I do not think that it is because the H&H is obsolete, it is just less desirable than the new rifle/cartridge, to that person. Others like the history, romance, nostalgia of a rifle/cartridge and base their choice on that. All fine! It is why we have a freedom of choice.
And no one should have to justify to another why he chooses one particluar rifle or cartridge over another, to any other person. It is their choice, to do whatthey want to do with their money, regardless of what someone else thinks.
For me, when I was having shoulder issues and recoil was a factor in the pleasure of shooting, I chose to build a 376 Steyr. A shorter cartridge that fits in a regular long action rifle, with less case capacity, similar performance, and could be built on the lh Ruger rifle I had as a donor rifle. The rifle is a pussycat to shoot of the bench, with the 260 gr AccuBond at 2632 fps out of a 21" barrel, and prints 1/2" groups. It has performed well on elk and bison to date. Hope to use it in Africa someday, but if not, it will definitely see more service here in north america.