If a commercial auctioneering firm did that sort of B!@##hit their reputation would be in the toilet where it belongs.
Evilbay has plenty of options available to sellers to "test the market" you can place whatever opening price you like or even set a low price then place a reserve on it. Making wholesale changes to listings part way through an Auction is apart from bad form the sort of behaviour that gets people in front of consumer affairs departments if done commercially so why should a private individual be treated any different.
An Auction once started with a valid bid or bids is the commencement of an agreement to sell an item to another and there should be consequences if the sellers backs out even if sideways !