We evaluate approaches to estimating demand elasticities in dynamic asset markets, both theoretically and empirically. We establish strict, necessary conditions that the dynamics of instrumented asset price variation must satisfy for valid identification. We illustrate these insights in a general equilibrium model of dynamic trade and derive the magnitude of biases that arise when these conditions are violated. Estimates are severely biased when the instrumented price variation is persistent or predictable. We then propose an approximate bias-correction factor and establish when such a correction is feasible. Empirically, we show that commonly used instruments yield elasticity estimates that are off by orders of magnitude, or even have the wrong sign. Our analysis further reveals significant shortcomings of standard multiplier calculations, and instead characterizes the dynamic asset market interventions required to sustain a targeted price support process, with direct implications for policies such as Quantitative Easing.