This article is from the eBay FAQ, by William B. Shaw email@example.com with numerous contributions by others.
There are very few legitimate reasons for retracting a bid on eBay. With
that said, there is nothing to prevent a bidder from retracting their bid
for any reason (or no reason). A count of bid retractions you make are noted
on your feedback page.
Excessive bid retractions can be seen as suspect (see Shilling) as the user
could be bidding & retracting high amounts on items just to find out what
the present high bidder's proxy bid really is - that practice is against
eBay's rules. There appears to be little, if any, repercussions for
retracting your bid. The seller has no say in the matter and feedback can
not be left either way.
More specific information can be found at eBay's page: