If you think about it, in a way you are trading your cards. You are putting the cards up on the site (hopefully), earning store credit for selling them, then using that store credit to purchase other cards on the site.
The things people buy for cash and the things people swap in trades are two completely different animals. As I mentioned before, I used to own a large collectibles store where I did millions of dollars in trades. People would trade for stuff they'd never buy in a million years. People would trade in stuff they'd never think to offer to sell to a dealer.
When buying, people tend to favor inexpensive items - stuff under $5 sells great for cash. They also favor items which are selling for deep discounts to guide.
When they have trade credit, they do just the opposite. They go for high dollar items selling at full book. Any collectible worth $50 or more is great trade bait. Offer it for trade, and chances are it'll be gone in a week. However, if you insist on getting full price in cash, you might sit on that same item for years.
I've dealt in comics, cards, toys, games, video games, cd's, dvd's, movie memorabilia, you name it. The people who are the most enthusiastic traders are sports card collectors. They're called "trading cards" for a reason. A person who collects toys might only buy the toys they actually want, and might never have any toys they want to trade. A person who collects cards constantly picks up cards that have value which they don't want to keep. They might open a pack and get a duplicate, or a player / team they don't collect, or an insert card they don't want, you name it. They might buy a collection or a bulk lot and only want a few items. They might run across a $10 card selling at a show for 25 cents. They know the card has value, but they don't want that card.
If COMC had an easy-to-use, inexpensive mechanism for hassle-free trades, I think you'd soon see tens of thousands of dollars, if not hundreds of thousands of dollars in trade PER DAY. And once the mechanism is in place, I think you would see an avalanche of submissions, of people who post their cards to the site just to trade them for something else.
I'd also point out, that this is another competitive advantage you'd have over eBay. eBay refuses to allow its users to trade, or to even mention the word "trade" in their listings.