we will assume that for any random variable $X:\Omega\rightarrow\mathbb{R}$, some investor will be willing to buy and some investor will be willing to sell a 'financial instrument' whose final payoff is $X$. (Actually, this is one of the few assumptions about the market that we have made that is actually plausible.)

This quote is taken from Steven Roman's "Introduction to the Mathematics of Finance Arbitrage and Option Pricing", 2nd edition, Springer 2012.

Why is this a plausible assumption? (The market model under discussion in this part of Roman's book is a finite model (finite time, finite probability space, finite number of assets) with no arbitrage opportunity.)

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    $\begingroup$ "The capitalist are willing to sell anything, even the rope we will use to hang them" - Lenin ;) $\endgroup$ – Alex C May 8 '16 at 16:11

You can think about them as noise traders in the sense of Glostem and Milgrom (1985). It it is a fairly wide used assumption that there is someone out there that soaks up residual supply/demand.

Usually one thinks about this guys as large mutual funds or pension funds.

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