This is already covered in our FAQ, but people have apparently glossed over it.

If you have a legitimate need to edit your bid (such as an obvious typo), you need to get approval from either the person running the auction or a mod/admin before editing the bid.

You will be held to any bids that are edited without permission. If you refuse to honor that bid, you will be temporarily banned from participating in auctions.

Please read the rules for this section before you bid!

http://www.nintendoage.com/pub/faq/NA/rules_forum.html#naa