Unfortunately, for heavily-traded items the marketplace is nearly unusable, because there is no way to specify a market-price or limit purchase. Instead, each item has a list of offers to sell from specific users (and I think each offer is for a single item, even if they have multiple they want to sell--- I certainly didn't find any way to make a bulk offer.) So attempting to purchase at a low price is an exercise in frustration, because anything on the first page is already sold by the time it pops up on your web browser.
The "feature" of "I want to buy from my friend" thus interferes with the main goal of a marketplace, which is to match up buyers and sellers. I dunno, perhaps they anticipated this and it's a mechanism to encourage trades at above-market prices (so that their transaction fee is higher.) Or are they copying somebody else's RMT market design which is equally bad?
Note also that the marketplace is saved from being a money-laundering tool only by the inability to cash out Steam credit. Transactions with named counter-parties rather than anonymous ones are just the wrong model here.
A set of cards can be turned in for a "badge" which gives "experience points" and other collectibles. The other collectibles trade at a significant discount to the cost of the badge itself in cards--- for the summer promotion badge, the 10 cards trade at about $0.15 each, while the two items (profile background graphic and special chat emoticon) I got with the badge trade at $0.03 each. (They seem to suggest some badges will provide DLC or coupons in the future.) An interesting experiment--- obviously the badge in the abstract is worth more to people than the specific rewards, and I don't think it's the XP. Though more XP gets you more "friend" slots, I guess that might be worth something. But I wonder if people think the badge rewards are better than they actually are.
(Yes, I did make a RMT trade to get the badge, but only as a way of trading my duplicates for the one in the set I didn't have. So I guess the badge--- or at least curiousity--- was worth at least $0.15 to me.)