Online marketplaces significantly influence consumer decisions, raising concerns about market fairness and consumer protection. Large platforms that control product rankings and personalized recommendations can shape consumer choices, while smaller or newer sellers may face structural disadvantages. Emerging online payment methods, such as buy now pay later (BNPL), encourage consumers to make immediate purchases by leveraging consumers’ temporal biases. These dynamics underscore the tension between market efficiency, profit maximization, and equitable treatment of all market participants. This dissertation investigates how consumers' decisions are impacted by algorithmic product rankings and financing options on online marketplaces. Using a combination of empirical analyses, theoretical modeling, and lab experiments, the three papers adopt an interdisciplinary perspective from operations management, behavioral economics, and marketing. The first paper examines fair competition in cross-channel sales, linking product ranking and pricing strategies. Using a hotel search dataset, it quantifies how platforms can use ranking to affect consumers' purchase decisions and explores how sellers can optimize revenue through pricing and commission decisions. The results indicate the power imbalances between platforms and sellers and provide evidence for regulations on platform commissions. The second paper studies consumer financial health in the context of BNPL loans, which lower upfront costs but obscure interest rates and future obligations. Using data from 5,019 consumers and a mixed multinomial logit model, it estimates consumers' time discounting and shows that BNPL is often perceived as cheaper than paying upfront, even when costs are substantial. A lab experiment tests an intervention displaying interest rates alongside monthly payments, demonstrating that cost transparency can reduce overconsumption and encourage more prudent financial decisions. The findings provide actionable policy implications for regulators seeking to address consumer vulnerabilities inherent in BNPL products. The third paper develops a heuristic optimization method for a travel subscription service, helping consumers navigate complex usage rules to maximize subscription value. This work illustrates how technology can enhance consumer decision-making while benefiting firms. Together, these papers advance the understanding of how algorithmic systems and financial products influence market outcomes, offering actionable insights for policymakers, platform designers, and firms seeking to create fairer, more transparent, and consumer-friendly online marketplaces.