I played a LOT of MTG years back and even bought several boxes to sell off individual cards in the hopes of making a profit. This site would have been insanely useful at the time. I lived on ebay and checked those prices against other individual sites for prices differences to get the best deal when building my decks.

I only glanced at the site, but it looks well put together. I am thinking about getting back in MTG, so I'll check this out if I start building decks again and buying/selling cards.