After you list an item on ebay you are free to change any of the terms of the auction - price, description, shipping, etc, until the first legitimate bid comes in.
As soon as someone has put down a valid bid, the listing is "locked down" and you cannot change anything. You may be able to add pictures, but I think that's it.
There have been several times that I have changed something, particularly prices, both up and down, after I see how the market is moving. If I had listed something at $20 and saw a similar item sell at $100, I might well be tempted to raise my starting price to $50 or $80.
