The problem is both simple to state and complicated to explain; there is too much gold in the game.
Single items are very well capable of being worth more than one gold stack (1 billion gold) when they are already tiered and plussed. Sets of items where that is true should be sold/traded together in one trade and can cost billions of gold. In the past month alone, I was told a set of items would cost 2.5billion gold and another single item would cost 1.3billion gold. This isn't unusual for some items imo.
You can't list these items in the auction house because you don't want to list items for less than what they are worth. You physically can't list them for more than 1billion gold.
Having a max gold stack or close to it isn't unusual if you have been endgame for a long time, and being forced to use alts to store gold when you go over is a nuisance.
Without knowing the statistics (please share, GMs), I assume there is more gold entering the game than there is exiting the game, and has been this way without fail since the start of the game. This is the reason all the other problems exist. If a significant gold sink was introduced years ago, none of this might be a problem, but I feel it is too late for a gold sink to completely reduce the prices to where this won't be a problem, but it could stagnate it and prevent future problems from occurring.
The solutions/suggestions come in three parts.
1. The obvious suggestion to relieve some of these problems is to increase gold cap. Unfortunately, I think this is an imperfect or temporary solution. I believe ROM uses 32 integer storage for values such as gold amount. That would mean 2,147,483,647 gold would be the hard cap. This isn't perfect or a long term solution. This is would at least allow us to bid on items in the auction house for that amount, though it might not be necessary to do...
2. Create a NPC that buys and sells an item (a gold bar?), for one million gold. Have the stack limit of this item be like 25,000 or something ridiculous. Allow players to trade this item to other players. This would allow players to trade other players for items higher than the gold cap. Want to sell an item for 1.5b? Trade another player and have them offer 1,500 gold bars, then go to the NPC and sell the gold bars. It would also allow us to store gold higher than cap while not using alts.
3. Create a gold sink. Anything that would make endgame players throw money at an NPC for a beneficial reason (as opposed to a consequential one).
Like, there was an NPC in Kampel Town in Land of Malevolence when it first came that allowed us to get like tier 4 and 5 runes from purple instance gear and gold. It was removed shortly weeks after release. Change that NPC to give tier 4 to 8 runes depending on the level of gear and gold amount entered and put it in Varanas or something.
Another item sink: Allow the listing of items on the auction house with no price listed to receive offers on the items for a fee so people can message them or mail them about the item to offer on it. This would allow people to list items for sale that cost more than gold cap. Charge like 10 to 25m per item to prevent or limit it from being spammed with items of no value.