What can't be proven one way or another is if an employee does or doesn't get a privilege that outsiders don't get because they are working for the auction company. The smart thing to do would be to avoid this line of questioning and just not have your employees bidding.
The other question that comes up is : how do you stop employees from bidding on names ? They can all have spouses, friends or other 'proxy agents' bidding on their behalf. . . Really there's no way to stop it .
Think about this as well : if I'm in charge of the aftermarket or an employee (likely rewarded on performance of the area I manage) and sales are a little down I would sure know of an easy way to boost my sales numbers. I'll just bid.

Why wouldn't I . . . I meet my quotas, I keep my job and I get some good domains.