Here is how I do it and I currently serve 15+ people using smelters as a point of silver transaction.
1. Make guild island, be the owner or atleast co-owner.
2. Invite the player you want to exchange silver with to your island, give them resources to build a smelter.
4. The player builds the smelter.
3. Remove their building rights, and claim the smelter.
Now what happens is the player is the initial builder of the smelter so he is kindof a co-owner but just that smelter.
If I unclaim the smelter - he can claim it without island co-owner perms, at the same time I can claim the smelter at any time.
Meanwhile, other player WONT be able to claim the smleter if its unclaimed cause they are not co-owner on the island or builder off the specified smelter.
Once a person is done making trades with me - I simply destory their smelter and build a new one once a new person comes.
I would add to the suggestion this: we need a track or record of withdrawals in every building. We use this mechanic as a guild sometimes to administrate different budgets so it would be helpful.