It could work but I see it having the most potential for trading away junk cards that no one actively seeks rather than in-demand cards. For cards with normal competitive value, you can pretty much get other cards normally by trading. Buyers won't purchase a card from the auction if they can simply get it from trades at its normal price. Sellers won't be willing to sell for less if they can just trade it away at normal price.

But it could work for trading away junk cards to find other junk cards to complete one's collection. The main reason why people are discouraged from putting up trades for cards that are not sought after is the token fee and the fact that they have to specify exactly what they want to trade the card away for. This typically results in a poor card-for-card match and the trade usually expires. Having an auction-like system is essentially posting a trade but allowing the "trading for" section to be open to all offers. That way people can offer junk cards they don't need for junk cards they could be looking for, typically for set completion purposes I guess. It's a way to get rid of junk cards that would be too impractical to set up a trade for.