1) You can retract a bid you have placed if you put it in error.
Eg. You typed £99.50 instead of £9.50, or you realise you bid on the wrong item, etc.
Bids can be retracted right up to the last 1 hour of that auction.
More info and the place to retract your bid can be found here:
http://pages.ebay.co.uk/help/buy/que...tract-bid.html
2) Ebay seller fees vary according to 2 things:
a) The amount the item is listed at.
b) The finishing price the item goes for.
So lets say you place your item at auction with a starting price of 99p you will be charged 15p. If you start it at £5.50 you will be charged slighly higher, eg. 30p.
There is a chart detailing costs here:
http://pages.ebay.co.uk/help/account/fees.html
You are then charged a set percentage on top depending on what price your item finishes at.