I wrote this before @macdguy amended that only certain items would be Marketplace-able. Take that as you will.
I personally think this is an alright decision. As posted in the thread, and talked about in other places ever since trading was announced, there were always ways people where going to trade items for units (often for real world currency) from the get go.
Putting all this on the Steam Marketplace makes all that thinking redundant. The great thing about it in my mind, is developers get small cuts of every transaction on the Marketplace (all be it a small one), which can help would with development costs as well.
I also want to put in here, that the prices of said items would be strictly governed by supply and demand by the community itself. So the community would end up deciding what items would be priced at what, simply based on the demand for said item.
Looking at this from all angles however, I do see how some people can see these as either micro-transactions, or even a bit unfair. I can see some people arguing "Oh, well you can just buy "so and so item" on the Steam Marketplace so why would anyone play the game modes". In my response to this completely made-up complaint, I would argue that people play game modes to have fun with friends more than they do to grind units.
If someone doesn't want to play the game worlds and just decorate their condo, and they want to spend their Steam Credits on items, I think by all means let them. Let people play the game how they want to play it.
Another thing I can easily see happening, is people buying items with real world money, then turning around and selling it for Units, effectively creating a "buy units for real world money" situation. Again, I personally think if that's how someone wants to play the game, then that's how they want to play the game. I can't really see it affecting anyone.
So to end this post off with my personal opinion, I'm all for it. It doesn't really affect the game in anyway in my mind, and something people need to remember is, at the end of the day, you don't have to use it.