The only real reasons that ebay accept for retracting a bid is:
* If you bid the wrong price (2000 instead of 20.00) - but you've gotta put in the correct bid afterwards or ebay may suspend you
* If you tried to reach the seller and couldn't contact them. This, IMO, is a stupid reason, because if you have questions on anything you should ask them before bidding. Bidding is a binding contract! Ebay even tells you this after you hit the place bid button and asks you to confirm.
* If the description of the item has changed significantly. This shouldn't really ever happen, because once someone places a bid the seller can't change the item description except to add on to it.
Retracting a bid after a change of heart, sleeping on it, etc. isn't acceptable, and if you do it more than a few times, you'll probably get in a trouble. The number of bid retractions you have shows up in your feedback profile.
Sometimes if you REALLY have extenuating circumstances, a seller may cancel the bid for you. Or, if it's a seller you deal with a lot they may cancel it if you bid on the wrong item, for example I just did this the other day- someone wanted the XL size in an item I sell and accidentally bid on the medium.
As a general rule, i won't cancel a bid for someone, I make them do it themselves so that it shows up in their profile. One or two won't make a difference, but if someone's abusing the system I want them to get caught.
Sorry if I sound like a tough ass about this, but as an ebay seller I deal with this stuff EVERY single day