I'm going to agree with Michael on this. However now you have led us into a discussion of what is an item's value.

1) Michael says an item is worth what people will pay for it.
2) Professor Reilly <g> says that items have intrinsic value.
3) Marx says an items value is a measure of the labor that created it.

(btw Micheal is right)
However we do not really have to answer this question. The real question is what does Bob reasonably expect this auction to measure.

If the rules prohibit shill bidding then Bob reasonably expects that he will buy the item at the value determined by #1 above. In the example of the $20 minimum bid he risks the possibility that the value as measured by #1 may actually be lower than the minimum bid, but he knows this before he bids.

If the rules permit shill bidding then Bob would have no reason to expect that the value of his item was being measured by #1 above.

Bottom line is that if Bob reasonably expects that he is buying the item at the price that other people (at least those at the auction) are willing to pay, shill bidding robs him of this expectation.

I don't feel like I am on the the witness stand, but I just had a law school flashback.