*This paper aims to examine the relationship between marketplace design and seller competition on online platforms. Using a game-theoretic model, we investigate how various design choices, such as pricing strategies, may influence the likelihood of seller collusion and the incentives of platforms to either break or sustain cartels. Our findings suggest that high transaction fees may reduce competition between sellers by increasing their incentives to collude, and platforms may also have the ability to manipulate the variety of products in each category. Our theoretical framework also allows for the examination of additional design features, such as information disclosure and the number of products, and their impact on seller competition. Overall, this research aims to contribute to the limited academic discussion on how the design of multi-sided markets affects competition on online platforms.

**I study the optimal alternative to a carbon tax using already existing taxes in the economy and its impact on inequality. I find that there exists an alternative first-best to the standard carbon tax. Under this alternative, the carbon tax on energy production is replaced by either taxes on energy consumption or taxes on inputs. Taxes on inputs used in the energy sector act as a substitute to income taxes: the government can choose between making households bear the environmental tax burden or to indirectly tax the energy producer. In the latter case, the final good sector can be partly subsidized. Studying the optimal Ramsey fiscal policy, a consumption tax is needed along the carbon tax as long as the social and private gains of increasing energy consumption relatively to final good consumption are unequal. There also exists an alternative to carbon taxation in which firms are taxed on their inputs. In this case households do not directly bear the tax burden, but are indirectly impacted through income taxes. If inequality is not affected by alternatives to carbon taxation in a first-best setting, this might not be the case under a Ramsey optimal policy.