I'm almost terrified to use the Auction House at all.
That 10% AH cut means you just can't make any narrow margin profits. Selling a 25,000 recipe MUST be sold for more than 27500.
It also discourages trying to sell any valuable items, as posting an item worth, say, 500,000 costs 50,000 just to put up...
This only leaves trying to sell stuff in zone chat, which is just tedious, and you're likely to get scammed by someone changing numbers right when you press 'accept' on an offer, even though it's supposed to reset when something is changed.
So, are there any items that sell well on the AH, and that by being posted here doesn't drastically change the SK economy to shift into having a surplus of that item because everyone ends up reading this? I know some people have made quite a living off of buy/sell, but I'm not sure how they manage it with those fees on AH and energy trading.
First, you forgot the Bazaar forum. Along with Auction House and chat, that's another key part of the marketplace.
Second, yes, you can make money selling things on the Auction House. Here are two uncreative ideas:
* Try selling recipes that are not available in the Hall of Heroes. Those include new gunner gear and boss item upgrades. But you have to be careful, because some recipes (e.g. Dark Briar Barrage) don't sell well for some reason. Get to know the market, before you invest a lot of money.
* Some players buy Orbs of Alchemy with energy and then re-sell them one at a time for a higher price. But last I checked the 10% cut made the margins on this transaction pretty thin.
For more information, try searching Google for things like "merchanting site:forums.spiralknights.com", "auction house site:forums.spiralknights.com", etc. Several players have made guides over the years.