I think the proposed point was missed. It doesn't need one dealer. It doesn't need one /central storage. A group purchase could be nothing more than X stackers/dealers acting as if there were one entity. Starting from a premade agreement, target price and quantity.
Example:
Stacker 1 100 ounces
Stacker 2 200 ounces
Stacker 3 300 ounces
Stacker 4 400 ounces
Stacker 5 500 ounces
Agreement with dealer A: as soon as spot price is $X, process all our orders.
This dealer A based group buy is then a part of the forumwide group buy.
The same for another chunk stackers and dealer B. You just order at the dealer you want or even more dealers/divided. For ex, dealer A could say the other dealers 'I do this kind of coin', so that its easier to 'construct' monsterbox orders per dealer due to same coin kinds, or to benefit from eventual quantity reductions.
The crucial element is that spot price $X that is agreed upon.
Because that is the element that converts all these individual orders on individual times to one price/moment, as to 'surprise' the high supply chain with a price lock.
This is the crucial element that should give success: because ex on the Comex futures market, the price mechanism works at the resolution of 1 position take or dump. After every position change, the spot price is updated. But the physical market has a latency, and due to the price locks at dealer A (and B and C etc) level, the high supply chain is unable to adjust the price in time to inflict us less silver. At least that's my theory.
For the rest, nothing changed. Dealers make the same profit as they made without group deal. That agreed upon price $X is a moment determinator, it's of course not the price paid, and individual dealers can maintain the margins they had.