Here you raise a good point. It certainly seems wrong on the face, but to be fair, if a friend of mine is bidding on an item, I won't. And if I am bid against by a friend, it always leaves a bad taste in my mouth.

There are exceptions, the club auction for example. Many of the bidders are friends, and those friends are who I am in competition with.

I do draw a distinction in that individuals who agree not to compete with another, may not have intended to compete anyway. One could argue this hurts the seller and the buyer. But the "conduct" is passive.

When a shill bids it is active, and hurts all bidders. Weeding out one potential bidder (or even a few) doesn't eliminate all possible competition. Finally sellers havethe ability to set minimum limits on auctions as a safety gap.

Ultimately, this form of conspiracy (and it is conspiracy) is an agreement to not act in a given manner. This passivity makes less of an issue.

Although if I was ever the seller who was conspired against, I may feel differently.

Not an easy question, and one I will need to ponder some more-now I won't get any work done this afternoon.