Spiral Knights helped pioneer Steam Trading a few years back, and so I'm curious that such a game doesn't let any of its items be marketable. The game has revamped its system greatly over time as well, culminating in an update a few days ago that removed the daily (extendable/temporarily-eliminatable) limitations of playing entirely, so I'm wondering if Three Rings (and now Sega since they bought Three Rings) has looked into pursuing ideas with Valve concerning this, perhaps to help bring back exposure to the game.

There is a similar discussion that I started on Steam itself, and can be found at http://steamcommunity.com/app/99900/discussions/0/864975399581680159/ but I'll bring up the important points that have transpired there so far for relevancy:

By Leon Alabard:
"I don't think it will work properly, considering how the "bound inventory" system works. Right now, we can only trade (ingame or Steam Trading) unbound equipment (+ accessories), materials (with some exceptions), [Crystal] Energy and rarities from the cash shop.

Materials are simply worthless (even something for 500~1000CR wouldn't not be worth more than 3 cents) because it's not that hard to get them (except for Shadow Lair-exclusive materials, since you need to complete the Lair to get them, but the material itself is useless since the SL-exclusive Alchemy Machine can only be found in the Lair's last floor).
There is no reason to buy a rarity item for Energy, and then sell it on the Market, since you have to choose between sell it for a cheap price (so it's a terrible idea to you, since it is not profitable), or sell it for a price higher than the Cash Shop's price (so it's a terrible idea to the buyer, since it's cheaper to buy it from OOO).
Selling Energy on the Market wouldn't not be profitable for OOO, even with the 10% tax on each transaction. Making the consumer to buy CE directly from them is simply more profitable. And I think that selling CE for real money (like Paypal) is also against the rules.
I guess people could sell unbound equipment with decent UV, maybe some accessories, on Market just fine, though it would reduce the effectiveness one of the most important CR sinks in the game (the CR transation tax).

If OOO removed the "bound" limitation, the Steam Market would be interesting for SK, though I think that it would probably break the game's progression (even with the new equipment/rank limitation, because of how the new craft system works)."

By Me:
"Hmm... yes, most of that I can see wouldn't be very worth it, unfortunately...

Though, concerning bound equipment, it is possible to unbind them with Energy now, unless I'm mistaken, via Vise, the Unbinding Smith in Haven's Bazaar:

# Star: Energy Cost / Crowns' worth on trading based on value of 6,400CR for 100CE / $ worth from OOO based on Starter Pack of $19.95 for 7,500CE

1 Star: 100CE / 6,400CR / $0.266
2 Star: 200CE / 12,800CR / $0.532
3 Star: 600CE / 38,400CR / $1.596
4 Star: 1,800CE / 115,200CR / $4.788
5 Star: 4,000CE / 256,000CR / $10.64"