Normally sellers shill in order to make sure they get every last penny that you are willing to pay. If they try to get more, they end up selling to their own shill i.d.
But in the case of a few famous/infamous ebay auctioneers, I believe the shilling actually creates a feeding frenzy, resulting in prices that are sometimes higher than market value. Over time a feeling grows in the collector community (among some, but not all) that the auctioneer in question has superior goods, which only exacerbates the 'problem'.
Not sure what ill-advised aberration of a human trait causes some people to participate in such schemes, but it's proven to be an effective business models. Given that ebay gets a cut, I would not expect it to ever stop;if anything, ebay will simply implement additional policies to cover up our ability to prove that anything illegal is going on.
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