A performance analysis system analyzes the performance of affiliate sites that provide links to specific items in an electronic catalog, and identifies catalog items that can be listed by such affiliate sites to improve performance. An association mining component analyzes transaction data attributable to specific categories of affiliate sites to identify items that are frequently purchased in combination by users of such sites. The detected item associations are used to evaluate, for a given affiliate site, whether significant disparities exist between the expected and actual sales quantities of specific items. The results of the analysis are incorporated into affiliate-specific performance reports, which may include specific recommendations for improving performance. The disclosed methods may also be used to analyze the performance of, and provide recommendations to, online sellers within a marketplace or auction system.