The advent of decentralized trading markets introduces a number of new challenges for consensus protocols. In addition to the 'usual' attacks -- a subset of the validators trying to prevent agreement -- there is now the possibility of financial fraud, which can abuse properties not normally considered critical in consensus protocols. We investigate the issues of attackers manipulating or exploiting the order in which transactions are scheduled in the blockchain. More concretely, we look into order fairness, i.e., ways we can assure that the order of transactions relative to each other is fair. We show that one of the more intuitive definitions of fairness is impossible to achieve. We then present Wendy, a group of low overhead protocols that can implement different concepts of fairness. Wendy acts as an additional widget for an existing blockchain, and is largely agnostic to the underlying blockchain and its security assumptions, as long as they provide a known and always active set of validators. Furthermore, it is possible to implement fairness for some subsets of the transactions, and thus run several independent fair markets (as well as some unfair ones) on the same chain.