Get the items to be "marketable" on steam.
Stuff in Steam Market
At first I was thinking "nah this shouldn't happen", but the more I think about it the more it could work.
Simply put, unbound items could be sold on the market. The way market transactions work, a percentage of the money would go to Three Rings anyways, meaning they grab another way to get money thrown around.
However, probably the best way to go about this would be to make costumes, accessories... Basically all things that come out of promo boxes be marketable.
The issues that would arise would be rare costumes and accessories being sold only on Steam for no listing price and all that, cutting many other players out from the goodies. Players may be more willing to purchase the items in-game however.
Overall it's something to look into, but would need a fair amount of refining to make it worthwhile and reasonable to all players. Heck, it would likely bring in more "play the market" players who invest in a game to make money off of it.
There's no real reason to do that for stock weapons, such as anything that's not promotional.
However, things like the Soaker or Mixmaster might have a chance. Though compared to weapon prices for, say, TF2, buying something like that off the market would be a lot of money.