I'm not saying I know for sure they have the ability to list cards for free or a heavy discount, it's a guess. However, I figure with near certainty that at least one of those guys doesn't have the kind of money to list that many cards at their advertised rates. Either they have a heck of an employee discount, he got some sort of sweet promotional deal or he bought a number of large portfolios already listed.
I'm not sure if I see how staff selling their own items can make a that big difference with their business model though. They are there to do the "dirty" work of selling (scanning, listing and shipping) and charge a fee for it. They make their money off other people's listings. They don't set prices, although I suppose if they listed enough of any one card, they could help manipulate the price somewhat. They would only drive it down though, otherwise other sellers would undercut them, which doesn't help anyone but the buyer. I suppose they could also shill a certain card, by listing one at a "reasonable" price and then list others at an inflated price to make the one seem like a good deal. That seems like a lot of work for not much return.
The sellers set their prices and can adjust them accordingly though. The worst I see that they could do is undercut the rest of the sellers on items and increase their individual profit margins by not having the costs associated with listing that other otherwise equal sellers have, unless I am missing something or didn't think of a different angle.