Actually, calculating equilibrium prices of markets is PPAD-hard (similar to NP-hard, but not quite) in general. See for example
<a href="http:&#x2F;&#x2F;arxiv.org&#x2F;pdf&#x2F;0904.0644.pdf" rel="nofollow">http:&#x2F;&#x2F;arxiv.org&#x2F;pdf&#x2F;0904.0644.pdf</a>