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 disagreement -- 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 relative order
fairness, i.e., ways we can assure that the relative order of transactions 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. Furthermore, it is possible to apply a
the protocol only for a subset of the transactions, and thus run several
independent fair markets on the same chain
