Buyer's remorse can occur either immediately after the auction is won, often as the result of a bidding frenzy, or it can occur some time later when the item is actually received from the seller and is not found to measure up to expectation.
Buyers' remorse is not a valid reason for failing to pay for or complete an auction transaction, and buyers who do fail to follow through, no matter the reason, are subject to sanctions as a Non-paying Bidder.
Note that the nature of the eBay feedback system has also led to a new usage of the term buyer's remorse, referring to the unfortunate situation in which a bidder may find him or herself after either having forgotten to check a seller's feedback or having ignored a poor feedback profile prior to bidding.
