Thing is, these guys have to make a profit from somewhere. They will want to auction the vehicle for profit, so you'll never get the p/ex value you would from a dealer.
I suspect they offer more online to lure people along, but I know someone who got the full value from them a few weeks back. If you're honest on the website and declare all the scrapes and shortcomings, they'll generally give you what they said they would (which, admittedly, isn't a lot).