Tax enforcement is especially costly when market participants are difficult to observe. The benefits of enforcement depend crucially on pre-enforcement compliance. We derive an upper bound on pre-enforcement compliance from the pass-through of newly enforced taxes. Using data on Airbnb listings and the platform's voluntary collection agreements, we find that taxes are paid on, at most, 24% of Airbnb transactions prior to enforcement. We also find that demand for Airbnb listings is inelastic, driving three key insights: the tax burden falls disproportionately on renters, the excess burden is small, and tax enforcement is relatively ineffective at reducing local Airbnb activity.