Shill bidding, the use of non-genuine bids to inflate prices, undermines auction market integrity. Exploiting China’s online judicial housing auctions as a laboratory, we identify 2% of participants as suspected shill bidders, affecting 8% of auctions. They raise price premium by 14.3%, causing an annual deadweight loss of ¥570 million for homebuyers. Mechanism analysis reveals they create bidding momentum and intensify competition. We establish causality using a difference-in-differences analysis leveraging a 2017 regulatory intervention and an instrumental variable approach using dishonest judgment debtors. These findings offer actionable insights for policymakers and auction platforms to combat fraud in online high-stake auctions.

China’s housing purchase restriction (HPR) policy imposes administrative restrictions on households’ home purchase eligibility to curb speculative demand. We quantify households’ willingness to pay (WTP) to re-acquire such eligibility. The empirical results based on the staggered DID specification suggest that when local governments implement the HPR policy, the transaction prices of judicial housing auctions legally exempted from HPR increase by 18.91%. This HPR-exempted qualification premium can be converted to an estimate of 22.48% of the transaction price as buyers’ WTP for home purchase eligibility. The heterogeneity analysis also suggests that the WTP significantly increases when speculative incentives are stronger in the housing market. If policymakers in mainland China consider replacing the HPR policy with an additional buyer transaction tax like that in Singapore and Hong Kong, China, the WTP estimates can serve as the benchmark in setting the tax rate.