This paper considers the boundary that separates marketable from non-marketable items. First, it examines the issue of blocked exchanges, that is, exchanges that cannot and/or should not take place. Second, it proposes to synthesise the seemingly separate issues of blocked exchanges from a single perspective based on critical realist ontology. Finally, it tackles some scepticism and criticism that has been levelled against the idea that ontology can be useful in determining a market boundary.