My experience is if it's a low end card it seems to go for a fair bit less than the lowest listing - especially if there's not a ton of them on the site. I did two auctions of cards on comc that really blew up in my face and I got half of what I would have got had I just listed them and waited a few weeks. I actually like going in comc auctions as a buyer, though. If it's a $6 card and someone unwisely auctions it then generally you can get it for $4~.
I'm sure there's plenty of shilling going on with middle to higher end stuff. Lots of the completed sales I see on 130 point are clearly bogus(both auctions and best offer accepted). The prices are generally 30% higher than most comps are fake sales rather than shilled ones it still shows how desperate people are to prop up the floor.