I'm starting to build an ecommerce site that requires unique product selection and pricing for each customer. Can Magento handle dynamic inventories for tens of thousands of customers and still perform well?
Magento can certainly handle that many unique customers with custom pricing groups. It will cause the database to be much larger, and perform more queries. With proper hosting and resources, there is no reason it should not perform well.
Combine in a Full Page Cache solution, and an optimized environment, and it should scale well. This may require a dedicated server or robust VPS if your database grows large to maintain solid performance.