IANAL, but it appears that bidding on behalf of the seller may be illegal in some jurisdictions unless it is in the terms of the auction. (From
http://www.maineantiquedigest.com/articles/ethi0800.htm, which I found when doing a search on auction psychology.)
From an auctioneer's standpoint, it may be impossible to know that George is a shill bidding on behalf of Fred - the seller. Heck, I suspect this goes on all the time and it's only because it's done badly that people notice it at all. (And the auctioneer doesn't really care either, since they get their cut no matter who the buyer is.)
Now the purpose of a shill is twofold. First, ensure an item sells for at least a minimum amount, which shouldn't be necessary if the auction allows minimum bids and/or reserves. Second, to drive up the final price - play chicken with the other bidders.
Now everyone loves a good deal. Picking up that item for a tenth of it's actual value is one of the reasons people go to auctions. But, that sale doesn't make the auctioneer or the seller any money. Personally, I prefer a posted minimum bid to a hidden reserve. Why waste my time bidding on something when you won't sell it to me at my final offer? But I do understand that starting with a low initial bid will typically get more people bidding, and then the mob mentality takes over to drive the final bid even higher. Shilling for a minimum bid then makes sense.
What's more reprehensible is shilling to increase the bid. It's also more risky for the seller. Although they may make some additional profit, I might also decide not to increase my bid, leaving them with not only an unsold item, but higher auction costs. But a good shill can read the sheep and figure out whether there's more cash to be squeezed out of my pocket based on how fast I counter-bid etc.
Now, shill tactics are even worse when the auctioneer is the seller since they can therefore avoid the penalty in fees and sales tactics. The auctioneer will also know a shill is a shill and not just another interested buyer. That might be considered out and out fraud.