I think that a healthy way for high level weapon trade would also be so that one had to level the item to maximum heat in order to unbind it. Sure, it could also cost some crowns, or a little energy, but the full value in energy of the item is a high price to pay for the trade of an item (especially with increasing alchemy costs).

First, those who already were grinding for items would have to grind more (still allows for variety, and many of those players proved they were willing to invest the time rather than the money). This would lower the number of high level items going to market.

Second, many casual players would be uninterested in investing the large amount of time (and energy) required for leveling items to maximum heat, further decreasing the quantity of high level items for trade.

OOO would still get their energy (and therefore money), just in the energy costs of elevators rather than a lump sum, and players would be less outraged. The value of rare items would go up, as they would be harder to obtain, yet still buyable by those willing to pay large quantities of money.

The overall economy of energy would be the same, but the seller would get playtime out of the energy, rather than having large lump-sums on both ends. I think that many of the high-level players who are outraged at losing the ability to sell high-level items would appreciate this as a reasonable solution to the unbinding cost, rather than perpetuating the outrage that many have about the appearance of "milking the players for money".

More paying players = more income, and this solution will retain more players while still achieving the same amount of energy bought and spent.

To supplement this, the amount of heat required to level an item could be increased (as has been suggested here: http://forums.spiralknights.com/en/node/8834), either across the board or simply on lower tiers (if only high tier levels gave lots of heat, and one needed high-level gear to access them, players would have even more incentive to buy from the selling players, who are using up massive amounts of CE to level the items to sell)

There may be some ramifications of this I haven't considered, but this is just a thought, and I think an excellent compromise from more a more "lump sum" approach that seems to anger much of the playerbase.