What is the ruling on buying in bulk a large quantity from a merchant who deals in Riba (usury/interest), with the strong presumption that he will use the money for Riba?

It is not forbidden to deal with a merchant, whether in buying or selling, so long as the transactions are permissible and conducted through legally valid contracts. The sin of his involvement in usury falls upon him alone. The Prophet, peace and blessings be upon him, used to deal with Jews, even though they consumed usury. However, it is preferable to buy from a merchant who does not engage in forbidden dealings.