There have been several disputes regarding the on-line retailers who quoted a below-market-value price but refused to honor the “offers” by consumers. Taiwanese courts do not have consensus on whether the purchasing mechanism in the on-line retailing website counts as an offer or an invitation to make offers, If the said mechanism is regarded as an offer, the courts are unable to decide whether, the retailers may revoke the contract. Using economic analysis, this article argues that if consumers consent to the end user agreement that stipulates that the purchasing mechanism is only an invitation to make offers before making the purchase, then courts should generally honor the agreement. If, for whatever reasons, the purchase contract is considered valid, whether or not the retailers may revoke the contract should depend on the mutual mistakes by both sides, rather than on the unilateral no-fault condition on the retailer’s side. The Ministry of Economics has intervened in the consumer retail market by stipulating model contracts that all on-line retailers have to obey, and yet a few stipulations have gone too far and hurt all consumers in general, causing major on-line retailers to ignore such mandatory stipulations.