I'm not that familiar with blowoutcards, so I can't really make a comparison. But I'd say the listings are kinda like Amazon.. The sellers have to send the cards to comc, then they are scanned, stored, a listing is created(the seller sets the price) and shipped when sold...
http://www.comc.com/Sell.aspx
The one big upside, is that no matter who the seller is, the shipping will be combined because they are all coming from the same place.. The downside is that cards come without toploaders(any orders I've ever received, the cards have been pretty well secured and packaged, so it's not a big deal). Toploaders are available at 20 cents per card if you elect to have them..