Here is my solution: limit the amount of gold available to one character at a time.
Say there was a gold limit of 2000 (2k). That means, the most desirable of items could only ever sell for 2000g. If someone has over 2000g at the time, they will lose that excess of gold. But it is their fault for selling things so high, or for buying things for such ridiculous prices. It also means that any current items on auction being sold for 2000g 1b, or more, will automatically be removed from the Auction House. Again, people would lose money for selling things for 3500g (which I have seen) or for however much over 2000.
Another solution which I have just thought of: increase the amount of money to put up the items for auction. This is a bad idea, unless the cost for auctioning increases dramatically at, say, 1000g. If you try to auction off an item at 999g 99s 99b, you would have to pay the current cost, but anything over that would be undesirable. Say selling items for 1000g costs 10g for the auctioneer currently, to put up for auction, the dramatic would be 100g. Increase the cost to roughly 10% of the price on auction.
If viable, the second solution could easily work in tandem with the first solution.